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.

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.