I am running Sierra and Mountain Lion VM's with Parallels on my 2018 Mini and they work perfectly although I haven't tried (and wouldn't try) to use any Flash software. Anyway, it was hard to find a good tutorial for installing MacOS VM's when I set this up, so I just figured it out for myself.
For me, the easiest approach was to use a Carbon Copy Clone of the old computer - which was easy, since I already had these. With a clone disk connected, use the Parallels option to create a VM from a disk image. It will find the operating system on the clone and create the VM very quickly.
But the "gotcha" is that it will be a blank VM that boots from your CCC disk - just like you could boot a real Mac from an external disk. You could just use it that way, but I didn't want to always have the clone attached. So I used the appropriate old version of CCC to clone the external disk to the new Parallels virtual disk (which was empty). This worked fine and I could then boot from the virtual disk.
Now, if you don't have an old Mac to make the clone from, I suppose this solution won't help. There are ways to create a MacOS VM using an installer image, but that was not as simple as it sounded and I wasn't able to get it working.
