Error 41145 after joining database to availability group
I've seen that recurring error about joining the database when automatic seeding was used to initialize the secondary replicas.  My solution has always been to disable automatic seeding once the secondary databases have all been created and synchronized:
ALTER AVAILABILITY GROUP YourAgName
MODIFY REPLICA ON 'YourReplicaName'
WITH (SEEDING_MODE = MANUAL);

Those information messages are normal, but I haven't ever seen thousands of them in a row.  My experience is that you'll see several of those messages per database per replica when the AG connection to that replica is established.
I just started up some VMs in my home lab environment that have:

1 AG
3 databases in the AG
3 replicas total (primary, sync secondary, async secondary)

I ended up with 90 of those &quot;DbMgrPartnerCommitPolicy&quot; messages (10 messages * 3 databases * 3 replicas).