If you use say Proxmox and create your own macOS image and compile your own Opencore then you should be OK. You can do same on VirtualBox. (On a PC, cause I doubt older MBPs have the necessary stuff like VT-d.)

Of course, once Apple kills off x86-64 macOS it is going to become more complicated.

That won't be for a very long time. There's a lot of Mac Pros out there. They're still supporting the 8-year old 2013 Mac Pro, the trashcan one. They sold a lot of the revised Mac Pro to animation studios and such. I wouldn't count on support for those being dropped for at least ten years.