Have a look at Scalingo, it's a good mix of simplicity and maturity.

https://scalingo.com/blog/heroku-alternative-europe-scalingo...

This looks nice! Wish they had a no-credit-card-required version for educational purposes. For the course I teach we use Spring Boot, and life was good with Heroku till they discontinued the no-credit-card version, and then the only choice we had (with support for Spring Boot) was to move over to Azure, which works but is a bit overkill and complicated for our purposes. I guess we could just use Docker and then many more platform would become available, but I'd rather not add one more step to the pipeline if possible.

Yes. We are taking a stab at the entire infrastructure like Heroku did but with a focus on a coding agent-centric workflow: https://specific.dev