Looks like that still has downtime for a Postgres migration- you're suggesting going into maintenance mode and just doing a dump/restore. I've seen that take hours once you hit the terabyte scale, depending on hardware.

I've had pretty good luck setting up logical replication from Heroku to the new provider and having a 10-15 minute maintenance window to catch up once it's in sync. Might be worth considering.

You might also want to add a warning about Postgres versions. There's some old bugs around primary key hash functions that can cause corruption on a migration. I've seen it twice when migrating from Heroku to other vendors.