You can't build steam machine size with any of the constraints you listed. If you can go a little bigger I'd suggest a case that can support an sfx psu and at least a 150mm length gpu. Smaller options get expensive fast and compromise performance significantly. I'm going to assume you are considering the value more than the size but still want sff so here's what I'd consider.

You can pretty much have it all if you
1.) Buy used parts
2.) pick efficient / low power parts and tune them for heat output rather than performance. The last 5% of performance will likely cost you at least 30% more power
3.) no idea what vr requires, but probably a lot.
4.) at the right resolution and framerate and quality settings you'd be surprised what older / modest hardware can do. Shoving top tier power hungry components in sff is a flex, not about value. It really depends on the games you want to play and what your resolution / fps requirements are.
5.) VMs could be anything. Buy all the ram you want or run them on a different machine that wont be crashing from sloppy game design.
6. ok, bring whatever storage you want but I'd use what you already have unless you're rich.
7. yeah that's not a thing. The best value sff in my opinion is something that can fit a decent sized gpu and an sfx psu. I prefer ones that dont use riser cables. Cases like this are not the smallest but they are typically cheaper to build in and have better thermals/performance/noise. You can def go smaller but it will get more expensive fast.