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.)
All three of my kids were riding full bikes around 3 years old, having used a balance bike for 12-18 months previous. I don’t think my kids are exceptional - balance bikes work wonders!
Disclaimer: I was one of the founders of the business, but left a long time ago. It’s still a great service! We print a newspaper for our friends and family each year.
I don’t believe it’s ever been accessible for free. It’s just that ownership has moved from the state to a private company and now it’s difficult to make it open.
I know Dave personally, and I can assure you he’s read Seeing Like a State! In fact, if you spend more than 30 seconds in his presence he’ll probably give you a copy.
Our experience of moving from Heroku to CrunchyBridge has been very similar - excellent help with the migration including jumping on a call with us during the switchover to resolve a broken index.
Would strongly recommend them to anyone looking to move off Heroku.