I recently migrated a small Rails project (16,000 monthly users) from Heroku to Helm/Kubernetes, after having tried Fly.io and everything else. It was easy, and I like the peace of mind of zero-downtime deploys and non-beta infrastructure. Cost is comparable to Heroku.
My writeup, in case it helps others: https://www.vmii.org/blog/2023/03/12/kubernetes/