UWA'a are quite demanding to use; shrinking so much more into the frame, detail is reduced, perspective & distortion increased and all harder to see in the view-finder.
Likewise, covering a bigger area of scene, you frequently get a similar increase in tonal range and contrast... as well as unwanted or distracting 'clutter', to consider in composition, while you have more work to do to find an exposure that suits the range you have in frame.... hand-held incident readings can be helpful here.
As said, it's unlikely your 'phone has an angle of view much wider than the kit 18 at the wide-end..... and if you aren't using that lens, then I think you ought ponder why.
I know that the 35/1.8 is a gem, and compared to the kit zoom, is nicer to use, gives a brighter, easier to see image in the view-finder; normal angle of view crops a lot of clutter from the frame and concentrates attention, while lack of zoom gives one less variable to complicate matters.
The Kit 18-55... it's not as crisp or precise as the 35 in the results it returns; operation is a little 'toy', its not smooth or precise, and it's not as fast or bright... its NOT a 'bad' lens though, and for the entry level its intended, its more than fit for purpose; used with care and attention, results you can get with it should in so many instances be indistinguishable from much more expensive alternatives, certainly at typical monitor display resolutions, let alone web-resolutions.
To get that bit 'more' in frame where composition may be compromised by access... for the frequency it occurs? You have encountered the problem how many times? And you HAD a lens wide enough to do the job you wanted, in the bag or on the shelf!
If you had left the lens at home, and still wanted to get that wider shot on the spot; you could have turned the camera through 90 degrees to the portrait orientation and shot three or four sections to 'stitch' in post-process, which would have given you the same framing as you got from the 'phone.... and that technique, with either the 45 or kit 18-55 can make you pictures with as wide an angle of view or more, than you would get on one shot with an UWA. Seems the day for wide-questions, but have a look at my look at wides here:
Ultra-Wide-Angle vs Kit & Stitch, featuring a fish!.... looks at the scene coverage you get with a UWA compared to what you get with a Section stitch.. commentary in there suggests that wide lenses increase the demand on you to be diligent in their use, to pay more attention to composition and framing and exposure, and do so in proportion to the % of added scene area they pack in the frame for you...
If you aren't paying that attention & discipline now, with the kit and the 35, to even consider using the kit that would give you the wide you think you want... then looking for something even wider, is likely to merely add to your problems, not solve them.
I have the UWA Sigma 8-16 in the bag, because I like taking photo's at motorbike meets and shows, and with custom or classic motorbikes, parked up and folk milling about looking at'em and chatting about them; it's often impossible to get a clean shot of one without a UWA.. if you back up, you have people, rope-barriers or other bikes between you and the subject, while the angular difference from such a short subject distance plus people moving in periphery of the scene, makes stitching less practical and reliable or pleasant. Lens has some other uses too, but they are limited, and it's was an 'indulgence' purchase... I don't need to take photo's of motorbikes, let alone a £500 lens to take them with! But, its not so easy to get the results you expect with it; the lens doesn't do the job for you, in sort of the reverse way that a longer zoom gets you closer; it demands you do more to make it get what you want, and if you aren't prepared for that, it Is likely to be disappointing, not solving the problem you think you have or giving you what you hope it will do.
My advice would be to put the 45 and the 5500 away, and spend some time working with that kt 18-55 and get to grips with what it CAN do for you, rather than mourning what it doesn't.. get the absolute most you can from that lens, THEN think about what its real short-comings are.
While I have had mine; about four years now, it has been my most used lens. Mostly for the convenience, and usefulness of that normal mild wide to mild tele range about the normal angle of view. 55-300 is not so much used; and both 'expensive' fish-eye and UWA, seem to get even less use than the M42 screw-fit lenses from my film cameras, if put on it when I am getting a bit 'precious' about it all... I am actually contemplating treating myself to a 'normal-angle' zoom upgrade this year, on the notion that if I can afford to have a couple of £500 lenses sat in the bag most of the time, I ought to be able to justify at least that much on the one that's on the camera most of the time! And the Nikon 16-80, is current favourite candidate for that... offering a tad more at the wide end, an tad more at the long end, giving it that bit 'more' usefulness either side of normal angle, as well as better optics and nicer 'handling'... practically though.... nice as that may be.... it IS just a nicety, difference in optics I expect to be almost imperceptible to what I cold get with what I got, and a little more care, attention and diligence.... but, if I can afford it? What the heck, its all an indulgece...
For you... the Sigma 10-20 is a useful UWA, probably more practical than my 8-16, and half the price new a very good VFM wide lens, even more so at 2nd Hand prices.... BUT, if you are incing over the investment and NOT getting what you could from what you already got... and you want it to solve a problem you don't really have? Its spending money, to make more problems than you solve, and not tackle the fundamentals that are causing them.... you are looking to the equipment to do all the work for you.... not doing the work to get the kit to do the job.... without tackling that... you'll just have more kit NOT doing what you want, wont you?
