2020-11-04 16:34:47.635 UTC [894681-1] ERROR: canceling autovacuum task 2020-11-04 16:34:47.635 UTC [894681-2] CONTEXT: automatic vacuum of table "x.pg_toast.pg_toast_981540"
Based upon Googling, we suspect it is the truncation step of autovacuum and its ACCESS EXCLUSIVE lock attempt(s).
No, I think that would lead to different messages explicitly mentioning truncation. (or no messages in most cases, as I think those messages only get logged either for 'vacuum verbose' or when log_min_messages = debug2 or higher). So something else is going on.
Are these vacuums happening for wrap around? I don't know why else they would restart so aggressively. On the other hand if they were for wrap around, they shouldn't be allowing themselves to get canceled so easily in the first place.
You mention a manual vacuum resolves the issue. For how long does it stay resolved?