Installing macOS on a separate APFS volume https://support.apple.com/en-us/HT208891
“APFS makes it easier than ever to switch between versions of macOS, including a beta (prerelease) version of macOS.”
https://support.apple.com/en-us/HT208891
I've done this to install a beta, but not to try to install an older macOS version from a newer release. If for some reason, the Mojave installer won't launch from Catalina, then you could create a bootable Mojave installer:
Parallels is, perhaps not coincidentally, not as terrible at running macOS as some other solutions, but it's still fairly poor.
It's not. I've run macOS under a hypervisor with GPU passthrough and it's essentially as fast as on metal (I think you can see a single digit percentage difference in benchmarks). I don't think anyone has a soft 3D driver though for macOS guests. Parallels and VMware have both put a lot of work into getting some level of soft VM based 3D acceleration working for Windows guests, presumably because that's where the demand is. At the same time macOS has been built heavily around GPU for everything for a very long time now, initially as a way to improve even basic interface smoothness in Aqua back in the day. So it's a dog in pure software on a VM. It really needs a GPU.
FWIW, in the past VMware did officially have Mac Pros on the hardware compatibility list for ESXi (up to 5.5 I think? maybe the trashcan lasted longer). And that in turn would be legit for running a Mac guest. I've read you can sort of get it to work on other Macs like the Mini, though secure boot must be disabled and the T2 causes other issues. Otherwise you need to patch it, or go with KVM or some other alternative.
[1] https://notebook.yasithab.com/macos/disable-sip-in-macos-vm-...
I still think I'd rather dual boot though.