I have built Cuber (https://github.com/cuber-cloud/cuber-gem) a few years ago as a replacement for Heroku and now we use it to deploy all our Rails applications on DigitalOcean Kubernetes. Extremely lower cost, better performance, less bugs, better support...
Is Heroku that expensive these days? What are extremely lower costs? In my experience DO is hella expensive.
Northflank is by far the cheapest alternative