The Firebase suite has been _fantastic_ for developing our app, to quote one of the app developers: "[...] but from start until now, I feel we have saved years of shitty bugs, and that it is one of the best decisions we have done". We still use Firebase for the app, and that's probably not going to change in a long time.

We hit some snags when we started requiring a performant API, which is why we don't use Firebase for that any more, although we still use Firestore and a lot of other stuff for the platform.

As every other technology it's choosing what pains you want, and the current Firesbase pains are quite easy to live with.

If AWS is an option for you, take a look at the AWS Serverless ecosystem, particularly Lambda, Eventbridge, and API GW. I have worked with these (and other services) for large projects, and they work great, especially for Node.js based applications. Aurora Serverless V2 looks promising too, but I didn't have a chance to work with it yet, only RDS and DynamoDB.

Just mentioning ( I'm not sure from your question if you're aware of this or I'm misunderstanding something), Firebase is a suite of tools within GCP, and you can have Firebase things call GCP things within the same project and IAM framework without any issue.