Problem: Thunderbird's calendar loses the ability to update my Google Calendar. I assume this is Google messing with OAuth or Thunderbird specifically to "encourage" direct Google Calendar use.
Solution: Host my own CalDAV server.
This will probably give me two problems (or more).
@VileLasagna Hey, hey, looks like my NAS (a Synology) has a calendar with CalDAV support... since I work from home, I can probably make this work pretty well without opening up anything to the Internet at large.