Whenever possible, I like to keep macOS High Sierra on a separate HPFS+ formatted partition/volume alongside Mojave & Catalina. I do this mainly for recovery/troubleshooting/testing purposes among other things. Separate but somewhat distinct.
While the iMac 2020 5K only supports macOS Catalina and onwards, the Mac mini (late 2014) supports Yosemite (10.10.x) all the way up to Big Sur (11.x) and the future release of Monterey (12.x).
For a 1TB internal HDD installed on the Mac mini, I allocated 100GB for High Sierra (HPFS+), Mojave & Catalina (both formatted in APFS)
Attempting to restore a High Sierra HPFS+ formatted DMG image file to an HPFS+ formatted partition using Big Sur’s built-in Disk Utility only rendered errors. Thankfully, the Disk Utility provided by Recovery Assistant (Command+Option+R on startup) provided a workaround.
Recovery Assistance also provides the ability to define the Startup Disk. After the successful DMG restore, High Sierra was now listed alongside Big Sur. Once booted and logged in, downloading the Mojave and Catalina installers and running through them was a breeze.
Software Update functions as expected.