Sorry to hear about your travails. Maybe it is a failing internal flash (SSD) drive. This drive is also referred to as the internal "blade" drive. Read on:
I have a late 2015 27" iMac with a fusion drive and had been experiencing similar problems, really over the past year or two. Just seemed to keep getting slower and slower. Would get spinning beach ball for what seemed an eternity all the time. Didn't seem to be too related to any particular task. I thought it was worse with Mail and Safari, but probably only because those were the applications I used most often.
I tried everything: using a different browser, creating a new user account, scanning for viruses and malware (found a few, but probably not the problem), removing all peripheral devices, booting into safe mode, removing start-up items, removing other applications, and finally starting clean with a complete erase and reinstall (time consuming hassle). I reinstalled Big Sur *without* using a full time machine restore - instead just reinstalled the applications I wanted and manually copied back my data files so that I wouldn't just "copy back" the old problem.
Some things seemed to help, but problem just came back and continued to get worse.
Then my computer started to panic and randomly shut down. I could not pinpoint the problem to any particular activity. Finally, it refused to start back up. Would turn on but then hang during start up.
Making a long story shorter, I used various recovery modes (finally figured out that making external boot USB was best method - albeit, rather slow) and figured out that the SSD portion of my fusion drive was no longer being consistently recognized. I was finally able to force the computer to simply stop using the SSD altogether. I then erased the internal spinning HDD (SATA) again and reinstalled everything (again!) only on the SATA internal hard drive.
Now my computer works almost like it did when new. Really. Very snappy now. I almost forgot what it was like. Yes it is missing the benefit of the 128 GB internal flash drive portion of the fusion drive, but it works far better than it has over the past year or two.
I am no hardware expert, but now understand that SSDs have a finite lifetime. At five years old of heavy usage, my guess is that it reached its limit. Everyone talks about how spinning HDDs will always eventually fail. True enough (and always make multiple backups!). But so do SSDs, and it seems that in my case, the failing SSD seems to have been the culprit.
If all else fails, you might consider replacing your laptop's 5 year old internal SSD. Not the easiest of tasks, but doable. You have to find the correct SSD - Apple's have proprietary pin connections (of course 🙄) and most others out there will not work. You can look at OWC for replacement SSD kits. Another company to try is Fledging (in Alabama of all places) - very responsive to my questions and dedicated to Macs. I am sure there are others out there (I found one on eBay from China that is supposedly specific for my machine, but seems a little sketchy). The "fixit" website may also steer you to good video directions.
Be aware that some people report problems after SSD replacement for older machines that include hibernation issues, lack of TRIM support, and other things I don't know too much about (but which can supposedly be addressed). But then again, a lot of people report having made this replacement successfully. Do you homework first.
I have considered the same, but this task is appears more difficult on a desktop iMac. I am too chicken to do it now, but whenever I finally get a new desktop, I may try open surgery to replace both the internal ("blade") SSD as well as the internal HDD (somewhat easier). If it works, I might just get another five years out of it as a secondary machine.
Addendum:
(If you have a thunderbolt port, you could also try running the machine off an external SSD - but this would not be as cool a fix! And larger thunderbolt drives still pretty expensive. If you can borrow one, you could try this first to see if it helps make your Mac runs better though.)
