Another update on my issues - turns out, Apache was running out of workers to service requests. We’re using Apache as a reverse proxy in front of Mattermost. So this helps explain some of the intermittent problems users had with posting messages, pulling up channels, etc. After re-configuring Apache, things have improved even more.
Previously, this was part of the error message in mattermost.log with some requests and led me to the DB issue:
Error 1267 (HY000): Illegal mix of collations (utf8mb4_0900_ai_ci,IMPLICIT) and (utf8mb4_general_ci,IMPLICIT) for operation '='"}