It’s very easy, and compatible, to put your system on an LTS kernel (5.4) and ignore the later kernels. (I have always done this.) Usually LTS are 5 minor versions apart, so I expect 5.9 will be the next one.
Usually i wait until the next LTS is properly stable, then switch to it, but this time 5.9 will not have the nvidia340 driver, so I will stay on 5.4 as long as I can.
Eventually, X-org will upgrade to a point where the nvidia340 driver cannot work with it. Holding back updates of X might not be as simple as staying on an older kernel.
If it’s not practical to do that, I will try nouveau again, and hope that enough of its problems have been fixed by then.
If nouveau does not work well enough then, it will be time to buy new hardware.
I got my Quadro card very cheap, 2nd hand, so it doesn’t bother me.
Whether getting a completely new system or just replacing the video card, I will be inclined to get an AMD gpu. I’ve heard that nVidia has the best graphics in some ways, but I don’t play 3D games, and any old card seems to be adequate for watching movies.
(I also have a 9-year-old laptop with AMD E350 integrated graphics, and it plays 1080p videos fine.)