The Bokeh Effect you like is 'disassociated' out-of-focus back ground.. with pronounced high-lights.. depending on your own interpretation of what Bokeh should be.
Indoor issue is the close focus of a longer lens being greater than the subject range out-doors... compounded by the small space leaving little room for distance behind the subject to fall out of the DoF zone behind the focus distance.
Your camera, D750, Full-Frame sensor? Lack of crop-factor will mean that longer lenses, give a wider angle of view (than they would on DX) for indoors, so you should be better able to fill the frame at closer subject distances, but, longer lenses tend to give shallower DoF for any given aperture, and have further close focus distances.
NOW... before looking too closely at gear, lets look at technique.... Fast aperture primes are the go-to must have techno solution for 'bokeh' backgrounds... folk don't seem to know, or can be bothered to find out how it actually happens, and expect lens to do it for them.. and when that's not enough a fast aperture....BUT subject is all.....
Depth of Field... is a % of the focus distance; and a 'zone' 1/3 in-front, 2/3 behind the focus distance... NOTE, this is the trick... the Subject distance DOES NOT need to be the 'Focus Distance'....
DoF is a % of focus distance.... shorten the focus distance, you will reduce the DoF.... make sense? This little nugget then offers a 'trick'; if you want to throw a back-ground OoF, you don't 'need' or necessarily 'want' a razor thin DoF, from a long lens and or super-fast aperture.. what you 'want' is your subject 'in' the focus zone.... and your back-ground NOT in the focus zone.... DoF gets smaller closer you focus.... so DONT focus 'on' your subject! Focus infront of them! This does 'beg' turning 'off' the ruddy red dots and going manual.... no.. NOT manual exposure, that's only a tiny bit of the 'manual' control on a camera.. 'manual-focus'... turn off AF.... NOW you ca decide where to focus, and don't have to have the machine trying to ut a red-dot bead on some actual target IT thinks you should focus on.... you now CAN focs infront of your subject... DoF will consequently reduce 'like' using a faster aperture, and the back-ground will fall OoF far sooner behind your subject, any unwanted DoF 'wasted' on free space infront of them where there's nothing in the scene for it to 'resolve'.
It is exactly the same technique as used by Landscape photographers to achieve maximum DoF in a big landscape.... backwards! And it is known as 'Selective-Focus'... and it's a fantastic bit of 'technique', and using it, you can start getting 'exactly' what you want in focus, and what you don't, not. You don't 'need' such amazingly fast aperture lenses to start getting very good OoF back-grounds, and you can get much 'better' OoF back-grounds, without the little niggles of chasing ever razor thinner DoF to get it, and noses and ears gong fuzzy, because without breaking that red-dot target acquisition dependency and arbitrary 1/3 ahead. 2/3 behind, or understanding the Camera to subject to back-ground distance relationships, you wont get the degree of back-ground dissociation until you are using very wide apertures and getting razor DoF.
THAT bit of technique sussed, you can start to get decent disassociated back-grounds, almost at will, with relatively 'short' lenses and relatively conservative apertures... and exploit DoF to keep eyes and nose in the zone of 'acceptable sharpness'.
Leads then to recognizing that even if the DoF drops off at a few cm behind a subjects ears, at a focus range of 3 feet, the back-ground STILL has to be at least as far behind the subject as the camera is infront of it, before you start to 'see' obviously blurred disassociated back-grounds... let alone what you might perceive as 'Bokeh' effects....... which tend to need 'high-lights' in the back-ground scene to show that fuzzyness and provide high-light rings..... sun-light through hedges, will tend to give more of that 'effect' than an evenly lit wall or curtain.....
AND.. this is absolutely CRUCIAL to your objective; knowing what makes that 'bokeh' effect and what influences it, and how much is NOT in the kit in your hand but the scene infront of you .... longer lenses, faster apertures it will remain hit and miss whether you get any of it, and you are likely chasing disappointment.... learn how, and more importantly WHEN you 'may' be able to get it.. with the kit you got, and you stand much better chance of getting results that please you.
And BIG chunk of that will be in recognizing the sort of back-ground scene that is conducive to Bokeh effects.... and how the camera-subject-backgroud distance effects the degree of back-ground disassociation...
So, pulling that into the more confined space of an indoor setting; F that's the effect you hope to achieve.... a different lens isn't likely to serve it on a plate..... and even with a different lens, you are likely to have to start employing studio techniques, and setting up the shot, arrangng camera-subject-back-ground to get the sort of distance between each that is likely to help you get OoF back-grounds, then probably stage setting that, to get a back-ground that will more pleasantly OoF, and if you want Bokeh, contains the high-lights that will give you those bokeh rings....
ON TO LENSES!
In days of yore, studio portrait photographers did not use 35mm 'small format' cameras. To many even 120 roll-film 'medum format' cameras were a little 'small', and preferred large format sheet film cameras! So, even with a Full-Frame DSLR you are't at an enormous advantage or portrait shots and effects, even in the oft larger space of an actual studio, let alone a domestic kitchen or dining room! Here the crop-factor effectively stretching shorter lenses, and giving greater relative DoF at closer focus ranges, is something of an advantage, BUT, heed you are working against the odds in the margns of the cameras comfort zone.
50mm is/was 'standard angle' for a 35mm or full-frame camera. Anything shorter is wide angle, longer tele-photo. The Nifty-Fifty, that came as standard with so many Manual-Focus film era SLR's was always loathed as much as it was loved. As a sales feature, low f-numbers were something manufacturers chased almost as obsessively in the film-only era as they do mega-pixies now.... they gave wonderfully bright view-finders... which was handy when they were tiny, and had a chunk of glass and a mirror behind them dimming the view! And they DID with the small-format of 35mm go a log way to helping folk achieve the shallow-focus effects that were obtained with Medium Format cameras with MUCH more conservative apertures..... Just as a idea my Ziess Ikonta 120 'folder' has a 105mm 'standard' lens with a mere f6.3 maximum aperture.... that would tend to make even the rather 'slow' f5.6 at the wide end of a modern kit' zoom look 'a bit quick'... b-u-t.. lacking complicated multi-element design and even more complex 'zoom' movements....that would still focus down to under 3 feet.... without exploiting the bellows slide like a macro-rig!
However.... 50mm lenses were oft critasised for being a bit of a one trick dog as far as wide-open shallow-focus effects went, whilst being rather either-nor, for composition, and a bit too tight for landscape and too wide for portrait, whilst always offering rather 'boring' and unflattering perspective; the 'favoured' portrait lens for a 35mm camera was 'around' 120mm; at portrait framing distances, that tended to offer a slightly more flattering perspective for noses and ears...
You already have that range in the 70-300 and aren't getting the 'effect' you hope for, and struggling with near focus distances..... a prime may give you a slightly smaller near focus, and faster DoF razoring aperture, BUT... without tackling the topic at source, and learning to exploit DoF and breaking AF dependency to get 'selective' focus rather than simple 'shallow' focus.
Finally the 24-70 would cover all distances between a 50 and 85
With that sort of logic, I would start by getting some-one with a GCSE in Maths to check your bank-account. before dong anything! ;-) 85mm is NOT between 24 and 70! lol! be NICE... I could afford SO many new toys if it was! But still.
I think you are chasing 'gear' to get 'effects' when in this stance, you need know-how.
You list a few wides in your profile; but not the 70-300 you mentioned; so I don't know if you have a more normal range zoom? That would probably be the more appropriate, and a 24-70, 'ish' probably your best bet, rather than a dedicated 'portrait' or prime lens.
I used a relatively 'slow' f3.5, 35-70 'zoom' as my general purpose do it all lens for many year, including the 'portraiture' required of my C&G course. (all though I did use, similarly 'slow' f3.5, 70-210 or some of that) I gave away the f1.8 50mm! It was so seldom any use for much!
It's your call, and a moderate range 'standard' zoom, i the 28-80ish region, IF you don't have anything in that area already, is probably a better 'all-round' buy; and almost certainly best VFM for the amount of likely use you may get from one in other circumstances, AND for 'occasional' portraits, likely more than helpful enough, IF you have the know-how vis exploiting selective focus, without which anything else is likely to be no greater help, and potentially a lot of hindrance.
So I would REALLY advocate you go get the know-how, not another toy; learn to exploit DoF, learn to take charge of the camera and make it do what you want, rather than relying on the Auto-Focus, and other 'gadgets' to do what you 'hope' for you, by serendipity.
Selective Focus.. not shallow focus.. technique not toys!
Thanks in advance ...
