As far as I understood from other posts on this forum you lose a lot of video card compatibility with the newer versions of MacOS on the old 5,1 machines. I don't know if you have the 6800XT working, if you do then that's good.
A lot of software I use now is getting to the point it's not fast enough on the 5,1. The 7,1 works fine - but that's still an expensive machine when it is highly configured. It is a workhorse of a machine.
My AMD 6800XT dual card worked flawless for about a year in Monterey. Very unfortunate, there is no driver support for newer MacOS versions. So I was forced to either sell my GPU or freeze my machine on Monterey indefinite. Since several apps need and will need newer macOS revisions in the future, I have sold my speedy card and fell back to my trusty RX 580.
However, I have missed the additional gpu boost, so I did buy a new sealed in Box AMD Radeon Pro Vega VII workstation Card. This is getting flashed from my friends in Poland and will arrive this week. This card is fully supported in Sonoma and later and sports 16GB of VRAM HBM2.
For sure, the upgrade path for the MP5.1 is getting smaller, but the game is not fully over yet.
I was contemplating to transition to a 7.1, but as non professional enthusiast Upgrading a card that originally did cost 1899,00 Dollars is a very economical solution Compared to MP 7.1 in Germany.
currently used mp7.1 go for 3000 Euros, so spending a tenth for a sealed new in box gpu is a great deal.
The 7.1 scenario would have cost me north of 4k because of junk gpu in most used configuration. I really never complained of slow CPU in the 5.1. Till this day the speed of the 5.1 has been depending always on the gpu performance.