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.
Sorry, but telling people to take a logical backup of their database, and then download it onto their local work station is insane for a production application. First, a logical backup at any decent scale will fail, and second, I don't even have enough local storage to do that -- even ignoring the compliance issues with downloading a full copy of production data onto a work station.
For a company like Northflank, I'd expect actual production-grade documentation for migrating, not instructions that are only applicable to a toy app.
Some folks want to do that, others want to import a backup directly, some want to spawn a read replica and sync their DB. Different strokes for different folks, all supported on Northflank.
Crunchy Bridge will help you migrate. They did a great job for us. We had a minute or so of downtime to let the read replica catch up and cut across. The team knows Heroku well, and some of them built it. (No affiliation, just a happy customer.)
Seems to be the sticking point for a lot of people, myself included.