Size limits, and to some extend temperature limits due to the case form factor.

AMD cards should work equally well, but they will not have hardware Ray Tracing. They have slower software Ray tracing.

So if you need RT you should go with Nvidia. Otherwise I think most things are equal, although the AMD cards seem to be performing slightly better on the temperature front.

Yeah, all these failing/flakey RTX-3xxx Nvidia cards is getting a bit too much for me. Unless I can find a good place to get a good/dependable one (like my old MSI Nvidia GTX-1070 ... I have two and they just work perfectly) ... I was actually considering switching back to AMD video-cards myself.

And with Microsoft black-listing the Intel i7-7700k 4.2ghz (Kaby Lake - 7th gen) in my Aurora-R6 for Windows-11... a new Apple with macOS is even back on the table.

I found some affordable GPUs, however most of them seems to be of ~29cm length. Will it require some modifications to the case, or no issue? 

I have contacted the Dell sales team, perhaps they have some GPUs that are both suited for R9 and don't cost fortune. 

I got a complete R10 with an RTX 3080 last year in October brand new from Dell for around $2,000 USD.

It's worth shopping around and checking the sales, if you are looking for something that packs a punch without breaking the bank.

Video cards are very overpriced right now.