People here underestimate google app engine a lot. But I doubt if there is better one person Saas service out there.
I run a NodeJS GraphQL server in App Engine Flexible, and it is basically just like running it in a Docker container. It's also pretty trivial to run in Google Cloud Run if I so desired, there is even a tool to assist: https://github.com/GoogleCloudPlatform/app-engine-cloud-run-...
Basically, your code shouldn't really need to change at all, it's really just your deployment scripts and configs that need to be updated. At their heart flexible GAE and cloud run are both just running Docker containers.