It's
probably silly to post this less than 24 hours in to using the new mini, but here's the deal so far. It's working just fine. It's replacing the 2014 rMPB on the desktop.
I'm glad I had an old wired keyboard and mouse; almost forgot about that until I started the setup.

The mini once set up, paired well with my existing Apple BT keyboard, mouse, and trackpad. It dropped the connection with the mouse a couple of times but picked it up again quickly. I've had a Bose mini-soundlink BT speaker for a few years; it's always been cranky about connecting to my rMPB. Rather than fuss with that, I placed it near the mini and used the headphone jack to link it by cable; works fine.
I encountered an issue this morning that may be more related to Big Sur than the mini itself. I have a new 6TB Fantom external HD to use for Time Machine. This will replace the old 5TB Fantom that was the TM for my rMPB (yes the internal SSDs are only 512G but I have another 5TB external drive with at least 3TB of photo and other files on it). When I connected the new drive to the mini I got the expected message that it wasn't recognized/mounted or whatever. I clicked the "initialize" option which brought up Disk Utility, showing the new drive. But I couldn't get Disk Utility to *do* anything to the drive. I tried it all twice. So I connected the new drive to the old rMPB which is still running Catalina. When I followed the "initialize" option there, Disk Utility was responsive, and quickly "erased" the drive with the correct formatting.
The real test of this new mini for me, will be photo editing in Lightroom, Photoshop, and a few other apps. I won't be able to try that until probably tomorrow night. I don't do crazy heavy duty professional stuff, like stacking 40 image files 🤣 - so I'll be happy if it's as good as the old rMBP.