Mark Dadgar writes:
I’ve seen mentioned that it’s not as simple as dumping the sqlite3 database and restoring it to, say, Postgres because some of the data types are different between them (which seems like an odd design choice).
It's not really a design "choice". It's implied by using a object-relational manager (ORM, in the case of Mailman SQLAlchemy) for the database.
As for helping with the problem, I'm not a real SQL expert, but I think it's probably possible to come really close by dumping the database and restoring it to PostgreSQL.
Also, here's an interesting project:
https://pgloader.readthedocs.io/en/latest/
It's just the first open source one I looked at after searching for "migrate sqlite3 to postgres". I have no recommendation, but I'll note that this particular software does log errors of various kinds, so it's not just a quick hack -- they've thought about some of these issues. There may be better ones, or you may want to go with a commercial product.
Steve