My 2018 MM Bluetooth is all but useless, I had to stop using my beloved Magic Mouse and Keyboard with them. They work great on my 2019 MBP as well as my NUC. My Airpods also do not work reliably on the 2018 Mini.
I replaced them with Logitech MX Keys KB and MX Master 3 mouse with the Unifying Receiver and these work great. (I have the MX Trackball but don't like it, maybe I will grow to like it...)
My wife has an M1 Mini that I will inherit once her new M1 iMac arrives next month. She has been using a Magic Mouse and KB from the start and has had similar issues. She cheats however and leaves the KB plugged into the Lightning Cable all the time but since that is not an option with the mouse she dropped the Magic Mouse and reverted to an old wired wheel mouse we had.
I suspect I will use the same Logitech KB and mouse when I get the M1 Mini from her. Hopefully I won't be looking up BT issues with the M1 iMac's when hers arrives.
My WiFi is off on the Mini, it is connected directly to my switch with Cat6. I still cannot get reliable Bluetooth performance from my 2018 Mini with Airpods, Magic Mouse or Magic Keyboard.Turn off Wi-Fi. If that fixes it, make sure your minis can only connect to 5.0 Wi-Fi.