Super slow iMac won't boot in safe mode

A couple of weeks ago my 2017 Mac started running really slowly. It gets caught up doing almost every task. I've run Malwarebytes, Disk Utility repeatedly, Activity Monitor isn't telling me anything, I've unplugged peripherals, unplugged the computer itself, but nothing's working. I've restarted many times, but I'm unable to boot into safe mode (shift on restart) or recovery mode (Comm R). The only oddity other than the constant freezing and sluggishness is that after I've used Preview and closed it, it still shows as open on the Force Quit menu even though it's not in showing on the dock.

I'm running Big Sur 11.6.7 and using 1.07TB out of 2.12TB. My desktop only has aliases on it, my trash is empty, I have no start up items, and I've cleared the caches. Any help would be greatly appreciated.