Autovacuum Truncation Phase Loop?

Hello,

We recently upgraded a 17 TB database from Postgres 9.6 to 12 using
pg_upgrade. After this upgrade, we started observing that autovacuum would
get in a loop about every 5 seconds for certain tables. This usually
happened to be the toast table of the relation. This causes the performance
of the table to decrease substantially. A manual VACUUM of the table
resolves the issue. Here is an example of what we see in the log:

2020-11-04 16:34:38.131 UTC [892980-1] ERROR: canceling autovacuum task
2020-11-04 16:34:38.131 UTC [892980-2] CONTEXT: automatic vacuum of table
"x.pg_toast.pg_toast_981540"
2020-11-04 16:34:41.878 UTC [893355-1] ERROR: canceling autovacuum task
2020-11-04 16:34:41.878 UTC [893355-2] CONTEXT: automatic vacuum of table
"x.pg_toast.pg_toast_981540"
2020-11-04 16:34:45.208 UTC [893972-1] ERROR: canceling autovacuum task
2020-11-04 16:34:45.208 UTC [893972-2] CONTEXT: automatic vacuum of table
"x.pg_toast.pg_toast_981540"
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).

This behavior hits our pg_statistic toast table a lot. However, it is
randomly happening to other tables infrequently.

I know that in PG 12 there is the new vacuum_truncate option that we can
turn off on a table by table basis. However, that is a concern since this
randomly happens to different tables.

I am seeking to understand what the best practice is in this case.

- Should we turn off vacuum_truncate for all tables or just turn off
vacuum_truncate for the tables as they happen?
- Should we run a "VACUUM (TRUNCATE TRUE) table_name" manually for the
tables where vacuum_truncate is turned off?
- Is there another database level solution I am not aware of?
- Lastly, does anyone know why this started happening after our upgrade
from PG 9.6 to 12?

Many thanks,
Creston