I just wanted to second the recommendation above.
Keeping the need to run older software and newer software separate will lead to a much better solution than trying to figure out how to keep one device to control it all. In short, if you need to use newer versions of macOS to keep up to date and sync all your devices in the Apple ecosystem etc. get a new Mac. If you want to run older software, plan on using an older Mac.
Keeping the needs separate also can lead to smarter purchasing decisions. For example, if you plan on keeping your 5,1 around to play old games and run VM, maybe you can get by with a M1 Mac Mini for your new Mac rather going for the M1 Ultra Studio. Or perhaps selling your 5,1 and purchasing both the current Intel Mac Mini and the M1 Max Studio would be better solution for your long term needs than getting the M1 Ultra Studio.
Personally, I have been keeping my 6,1 on macOS Sierra, to maintain compatibility with some 32-bit drivers and custom external hardware. For keeping up to date with the latest version of macOS, I have a MacBook Pro. Not having to use the MacBook Pro to run the external hardware has meant that I have been able to buy a lower spec'd MacBook Pro that I would have needed to if I didn't have the 6,1 around.