And then FCPX began working - seemingly for no reason.
What I didn't say in this thread is that I had been running Disk Utility. Running Disk Utility was one of the first things I did immediately after finding that reinstalling FCPX didn't solve the problem. At the time Disk Utility was reporting errors of the snapshot; Disk Utility failed to fix those errors. Below is the error I was getting from Disk Utility:
error: Unable to perform deferred repairs without full space verification
error: Try running fsck against the entire APFS container instead of a volume
I continued running Disk Utility repeatedly into the next day. Somewhere in the middle of that chaos FCPX suddenly began working. So I quickly edited a video (yesterday).
I started FCPX today and it opened just fine.
I also ran Disk Utility today and there were no errors.
So, maybe the issue was related to the disk having errors. I don't know.
Whatever the case - I need to upgrade my hardware. I'm pushing my luck running a 2015 MacBook Pro.
Thanks for all the help.