@julian sorry for the late response, I didn't think to check this again. Firestore offers ways to reduce the cost and total number of operations, but there are a few caveats. Here's their recommended optimization solution https://firebase.googleblog.com/2021/04/firestore-supports-data-bundles.html
Data bundles just means exporting data to a json format, store it in a cache, and serve the data from the cache. This does reduce the number of operations, but defeats the purpose of firestore.
You'll have to handle the authentication process and can imagine the problem. I'd much rather pay for a database, FYI already did.
Firestore offers other methods to optimize usage like subscriptions, compound queries...etc. After taking all of this into account, firestore became extremely unattractive.
To your question about the next tier, I'll need to take a look, but my gut feeling tells me that this isn't sustainable for large scale.