Hi. I'm having a similar issue.
I've upgraded a Mac Pro 3.1 to Catalina using dosdude1's method and installed the Video Legacy patch but FCPX 10.4.7 still won't run and gives a message saying 'graphics configuration doesn't support hardware acceleration'. My graphics card is a ATI Radeon HD 4870 512mb.
Can I get a graphics card that will support & run FCPX, can someone please point me to a list of compatible graphics cards? And if I get a new card, how do I install it so the system recognises it and doesn't give me errors?
I need the machine to be able to render & build proxies overnight. Thanks.