I prepared my external SATA/USB-C SSD by connecting it to a USB-A port on my M1 Mac mini, which I know works reliably for booting, then using Disk Utility to divide it into a series of 100 GB containers, each of which will contain a different version of Big Sur. The alternative is to keep old versions in Virtual Machines in Parallels Desktop. SATA/USB-C disks may need to be connected to a USB-A port rather than USB-C.
Some external disks can only boot when physically connected in certain ways, e.g.M1 Macs can’t boot from an older release of Big Sur without setting that bootable volume group to Reduced Security using Startup Security Utility in recoveryOS.
This currently appears insoluble, making it essential to create each bootable system when that version of Big Sur is still current.