Update:
After extensive research and a lot of good advice on these forums I decided to “bite the bullet” and sprang for a Radeon RX 6800 card and installed it.
It wasn’t cheap, but it should future-proof my graphics for a while.
It isn’t a $2000 graphics card, but it works and I am getting reasonable frame rates (> 30) with reasonably high levels of detail, so I guess I’ll call it a win.