I also just recently switched to https://purelymail.com after hosting my own for several years via Mailcow and a cheap VPS. The cheap VPS was more expensive than the $10/year for Purelymail and I was only using it for mail anyway. Nothing to maintain now and so far very happy with how easy it was to move everything over.
Also, they don't charge extra for additional domains so if you have multiple it's still $10. Only been with them a few weeks so can't say much yet for long term, but great so far!
I was just thinking about doing something like to migrate some of my communities over (and was even planning on writing it in Python). Just ran it and it worked perfectly. Thank you, this saved me a bunch of time!
I ran into this at one point as well. You can unset the private instance flag manually in the database and restart to get up and running again.
First connect to psql:
docker-compose exec postgres psql -U lemmy
Then run an update to unset the flag:
lemmy=# update local_site set private_instance = false;
Which should update 1 row (only 1 row in that table if you're on a typical install)