We used firestore before.. got a bit tired of some of the limitations (latency, indexing). Cost-wise I don't think it's that different actually, but we aren't using much bandwidth, then self-hosted can be dramatically cheaper. Have to manage some details of course (zfs filesystem parameters, set up backups, config postgres etc.), but I found that stuff quite interesting and it's knowledge that will always be useful.
I've always struggled estimating my DB size, curious what you mean by your estimate.
A snapshot is effectively identical to a killed database server (due to OOM, or power outage), and database servers should be crash-resistant.
With PostgreSQL, checkpointing the database before taking the snapshot may make the crash recovery quicker when loading the snapshotted database.
Most managed/serverless options begin at hundreds of dollars per month. So, you get lots of companies either just handing over the cash or jumping through hoops to get something more reasonable. The latter is a stupid waste of time if you can afford the former. That's how Google and Amazon make money: they make the expensive option more tempting and the cheap option needlessly hard. They are not interesting in supporting frugal teams. The whole point is squeezing their customers hard.
So, this is potentially very nice if it offers some competition on the cost front. I'd certainly consider using this if it proves reliable. In fact, the whole reason I opted out of a relational database is the above. What I'd need is something that is reasonable in cost relative to the modest data I store and retrieve.