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Which leads to the question of what the _best_ GPU possible is. Best in terms of most reccent or best in terms of most supported.

Being a member of the "plz no old card" fraction in former times I know both worlds a bit. New GPU's are put onto market under time pressure. Drivers from the vendor itself are being written only for Windows (to some extent for Linux nowadays). Of course tech specs of new cards do look tempting, especially when reviews are good. But if you buy bleeding edge cards you are Beta tester, even on Windows... Believe me, I learned that. Having GPU hangs, game crashes, memory leaks, etc. is not so nice when the only thing you can do is wait for better drivers to appear (hoping it's a popular card the vendor does care for).

So I defected to the "best supported card" fraction. On x86_64 Windows/Linux this would be Polaris cards (like your RX580) and Evergreen cards still (as they were very popular). On FreeBSD probably Evergreen cards as FreeBSD re-uses old Linux drivers. Well, and that's it for Mainstream OSes with _good_ GPu support. On other OSes (Haiku, ReactOS, NetBSD, etc.) you get less in terms of GPU feature support or GPUs running at all. If you check out Haiku GPU support you will see that MorphOS and OS4 do provide better support.

So for a non-mainstream OS GPU support in terms of features is pretty good in MorphOS! Having a look at the bigger picture I would not complain too much about OS4 being one step ahead providing Polaris support. Though I doubt they are able to exploit all the features Polaris cards got.

Having done some el cheapo 2nd hand Linux-PC builds for friends I learned that you won't miss much if you use at least R600 Radeons. (klick) 2D/3D acceleration works and OpenGL 3.3 is good enough for most open source games, h264 GPU decoding works, resolutions up to 2K possible.

What benefits you do get from >=Polaris cards? up to 4K resolution, h265 GPU decoding, OpenGL 4.x
Which is certainly nice to have but no must for an alternative OS. Of course Hans De Ruiter did a very good job and a 'common card' for MorphOS/OS4 would still be good.