This literally just happened to me last week. Large(ish) library crashes on startup, as well as its associated backups. It usually would crash right on load-up, but sometimes it would allow me to make a click or two before it implodes. I trashed preferences, removed recently installed effects and generators, deleted all render files, analysis files, and proxy media. No luck whatsoever. The fact that it is inconsistent in when it decides to crash is interesting. I’ve saved the crash log, would love if you could take a look.
2013 MacPro, 10.15.7 Catalina, FCP 10.5.2, OWC Thunderbolt 2 RAID MacOS Extended (Journaled)