I don't use bluetooth input devides so can't comment on those, but I too noticed regressions in the last month or so with regards to AirPods.

They changed the firmware so that it can "seamlessly" move between devices but 1) it's not seamless at all - I actually disabled the new behavior and 2) I'm pretty sure something else changed regarding Bluetooth because switching between devices manually is slower and less reliable than it used to be before, and I bet this change is also responsible for the issue with input devices that you're seeing.

When my iPhone 12 wakes it breaks the Bluetooth connection between my iMac and Beats earphones.

I thought I was losing my mind - I've been experiencing the exact same thing with the MX keys/mouse over the past few months, and no amount of fiddling or reinstalling has been able to fix it. My scroll wheel regularly gets "clogged up" too, becoming unresponsive for 5-10 seconds, then dumping out all of my buffered scroll data at once in a furious explosion of scrolling. It's quite frustrating, and I've been looking forward to a fresh install of Big Sur to hopefully flush out whatever gremlins have been gunking things up.

Happens with Apple standard keyboard and mouse too. Restarting the computer AND accessories fixes it.

Same... and then about every 2 weeks bluetooth just dies taking the whole OS with it.

are you using 2.4Ghz wifi? Bluetooth uses a similar range and it gets interfered. If you have 5Ghz wifi try it, it might solve your problem.

Same.-