It depends really
Improved:
- AF in a different league
- IBIS
- FF 4K 60p video, and also 6K external RAW
- crazy high fps
Seriously downgraded:
- cheap plastic body
- pixelated low res old EVF from sony's junkyard
- short, uncomfortable grip. This is no 5D any more. No better than Sony now
24MP vs 30MP is probably closer than you think. 5DIV has hell of a strong AA filter meaning only sharpest primes can properly get through that. So you likely don't ever get anywhere near 30, particularly in the edges... R6II still has AA filter but it is less aggressive.
I either need medium res files for clients, where even 16MP is overkill, or I need full 50++MP and that's for high res printing. Think a wall,. Do you do that?
OK. So this is essentially WANT rather than NEED, and there is nothing wrong with that.
This is a big WANT, and that's likely one of the better options for RF if you can drop so much cash in one go.
I would argue that maybe you will find the new 24-105mm f/2.8 more useful because of the extra range in one package - very useful for family lifestyle shooting.
These are not lenses I own nor realistically consider due to cost, and then plastic build. That much money buys me a bag full of Sigma ART primes and lots more.
I would not expect them to be optically perfect, but that would mainly manifest on 8K sensor like R5. You should be relatively OK with 6K R6II.
Personally I shifted mine a while ago without even replacing directly; 35mm and 50mm Tamron and Sigma primes cover the gap. One thing it was actually pretty good for was events, people and lifestyle. But if you venture into landscapes and high res cameras, etc I can see why you'd want something substantially upgraded.
If you want a zoom in RF mount then you don't really have many option other than what was mentioned above and I'd personally skip RF 24-70mm f/2.8 unless you fancy extra heavy [correctable] vignette at ridiculous price.
But you also have options in Sony with excellent 24-70mm GM2.
Yes, it is a huge uplift for people photography. Huge. No more messy slightly OOF shots.
However, you should be able to use 5DIV in live view mode as a sort of mirrorless just without the latest tech like eye AF and IBIS.
Why do you need R5 unless you print on 4M wide wall or shoot 8K RAW video? Just because it sits higher doesn't make it right for everyone. R3 is another step up the food chain and is only 24MP.
Besides HK R5 is now sub £2.5K so actually not too far off R6II, but as mentioned R5II is probably on its way but landing nearer £4k mark to start with if pricing patterns repeat. I'd much rather get Z8 and jump over to Nikon then. but then I need 8K you probably don't. There is nothing else in it.
Again what is the point unless it is a stacked sensor without AA filter so an actual overall improvement? Somehow I doubt this. Just get the Z8 then.
Best body right now is Z9 or Z8, except for same old pixelated crappy EVF.
Best new lenses are mostly from Sony, Some from Nikon
If have you a lot EF glass like me then that's a factor, but you can adapt to nikon and sony also.
IF you fancy halo lenses like 28-70mm f/2 or 24-105mm f/2.8 then your options are truly limited to Canon RF.
Here is another option. Buy the older R6 mk1 at just 1399 as a second body, and bag a nice Tamron 35mm f/1.4 & Sigma ART 50mm f/1.4 or Sigma ART 40mm f/1.4 and maybe 28mm f/1.4 and you will have plenty of ££££ left for travel and fun and have more capable kit. R6 vs R6II is mainly minimal changes.
Come to the photography show in Birmingham in March, play with the gear, and make the decision on facts and feel