Agreed. The 2018 mini has been a bit of a horror show under Big Sur. Some updates would fix things, then broken by the next point release. Has been incredibly frustrating.Dunno, after the awful everything I had with MacOS 11 "Big Slurp", I have had nothing to say but "Thank God"
(or, if one wants to be really accurate, "Thank the software engineers at Apple") for the MacOS 12 betas which made my Mac Mini 2018 behave itself again.
Beta 7 and 8 on the 2018 mini have conclusively fixed the crappy bluetooth for me; poor AirPods pairing and dropouts while on calls and hit and miss Apple Watch unlock all now solid.
I was almost certain it was a hardware issue at this stage, but apparently not!