Which works out at $100 USD / year. You might think that's trivial, but when you start provisioning multiple environments over multiple projects it starts to add up.
It's a shame that Google haven't managed to come up with a scale to zero option or serverless alternative that's compatible.
setup a DB project , use same cloud sql instance for all DBs. Did that for years on non prod or experimental projects. $100 is a bargain for what you get in terms of resiliency
It is trivial to set up a database on GCP given that you know what you are doing and I would pay Google for that stability and support for setting up multi-tenancy and region.
Using Google spreadsheets as a backend will just cause them to charge everyone later.
Sheet Ninja isn't free. Even on their side, "free" does not mean what you think it means.
Sheet Ninja is 108 USD / year and has tiny capacities for every metric. SQLite is free and would stomp this in every aspect on low budget hosting. Even a tiny API that stores CSV would be magnitudes more efficient.
But what would scare me the most, is that google can easily shut this thing down.