The new version of the Standard Environment removed most limitations (and you can still use the Flex Environment when required):
https://cloud.google.com/appengine/docs/the-appengine-enviro...
And using the proprietary App Engine API is not recommended anymore: “We strongly recommend using the Google Cloud client library or third party libraries instead of the App Engine-specific APIs.” (source: https://cloud.google.com/appengine/docs/standard/go111/go-di...)
You can use App Engine with a standard MySQL or PostgreSQL database and S3-compatible object storage.
Like you, I used to avoid App Engine because of lock-in, but they radically improved things with gVisor.
Yet, I agree about the price, especially with the Flex Environment compared to standard Compute Engine :-(