AMD Ryzen 4000 desktops are coming soon, using the mobile APUs rather than the desktop line of 4000 processors. They are a little bit over budget at $300+ but they are going to perform excellently ranging from 4 cores / 4 threads up to 8 cores / 16 threads and very decent iGPU. If necessary this could be repurposed to suit a number of uses like a modest workstation or 1080p gaming or some virtual machines or a media PC etc.

https://www.cnx-software.com/2020/06/16/asus-pn50-the-worlds...

There are a lot of older, competent dual core machines available on Amazon very cheaply too. These are much cheaper but if you ever needed even one virtual machine it would be quite taxing. These machines are 4 or 5 years old so linux support is likely excellent.

https://www.amazon.com/s?k=small+form+factor+pc&ref=nb_sb_no...

These aren't strictly "SBC" but they're very similar. Basically laptop motherboards and soldered chips with potentially upgradeable storage/ram.

The thin client corporate PC idea is a good one.

What would be fantastic would be to have a recommendation where someone has a really good existing setup.

You might like asking on this subreddit, they use everything from little raspberry pis to large rack servers, often acquired at a very low cost:

https://old.reddit.com/r/homelab

I probably don’t know what I’m talking about, but maybe a high-end Raspberry Pi?

These platforms work in theory. In practice I think it’s still only been a few months since OpenGL ES 3 worked in RPi4. Things like this make me worry it’s not the best platform for a performant workstation.

I’m fussy. I want my windows to drag / resize / scroll at 60 FPS.

You had me at Raspberry. Although they are small and inexpensive, the latest version is quite powerful. I think the only hindrance might be powerful video. It has great video, but some people need (want) more.

Does that include open or closed GPU drivers?

Closed would be fine

The real sweet spot I’m looking for is a platform that’s supported entirely in the mainline kernel because this just seems to make life so much easier.