Thank you so much for sharing. These are very good things to check for every migration.
I don’t quite remember if it can happen in phpBB, but we often check if there are duplicate emails or usernames in the Users table. We also check if there are staged users, you wouldn’t want to migrate them as regular, validated users.
Depending on the phpBB version you could add a check to the code so you don’t have to manually delete the posts every time.
Although, it is good to manually review the deleted posts. Some admins may want to keep them (hidden, for archiving purposes) instead of permanently deleting them.