Transaction age approaches wraparound limit

A Postgres 13 cluster started logging WARNING: database "prod" must be vacuumed within 12000000 transactions and the app is up but you're told it may stop accepting writes soon. SELECT datname, age(datfrozenxid) FROM pg_database shows the main DB at 2.05B and rising. There are several aggressive autovacuum workers running but they keep getting cancelled, the logs show canceling autovacuum of table X to prevent deadlock style messages around a nightly batch that runs ALTER TABLE and bulk loads. Describe your triage, what happens if you do nothing, the emergency mitigation, and the prevention.