Interested to hear what the paying business customers who migrated away from Heroku think about their new home and the process to move to Fly/Render/Digital Ocean/whatever?
I've migrated one larger project with several containers / databases to AWS Fargate / RDS and a smaller one to Scalingo [0]. In both cases, I'm happy with how it turned out and the migration went fine.
One of the main drivers behind the decision was that Heroku required us to move to the Enterprise tier to run private databases.
If the architecture of your application is not too complex, I would suggest trying Scalingo (or Fly.io, Render, etc.) first. It took me around a day to get the application running on Scalingo, Fly.io and Render. This allowed us to be able to compare them a bit. In the end, we went with Scalingo, since the Review Apps feature to automatically launch environments from pull request worked well for our workflow.
You can see the configuration for Scalingo for a smaller elixir app on Github [1].
