I solved my issues by doing a few things. (iMac 5k Late 2014 going from High Sierra to Big Sur 11.2) I manually removed as many old / unused and incompatible / 32bit apps and audio drivers etc etc as I could find that might be somehow conflicting with the Big Sur architecture. I downloaded Litte Snitch and blocked ocsp.apple.com from within the trusted folder I removed all USB peripherals such as USB audio interfaces etc / printer / USB drives and performed a SRAM reset on my main internal SSD. I also have a secondary internal SSD so booted from that and performed another SRAM reset on it, too. I'm no wizard, so I don't know exactly what fixed everything ––but doing this combo of things solved all my horrible Big Sur issues. Thought I'd share in case anyone wants to give these things a go :)