i'd recommend checking serverless framework (serverless.com) or openfaas (openfaas.com)

best thing you can do is not get involved with provider-specific APIs: use Docker/Kubernetes for building and executing your code, Postgres-compatible database (Hasura if you want Firebase experience) and S3 for object storage, send e-mails using SMTP

again, don't use provider-specific API's

If you run k8s clusters anywhere, OpenFaaS and KNative are both solid options. OpenFaaS is seems better suited for short running, less compute intensive things. Whereas KNative is a great fit for API's.. it just removed a bunch of the complexity around deployment (like writing a helm chart, configuring an HPA, etc).