Are you still with them? If yes, would love to hear why. Otherwise, what made you jump?
It is hard to replace GCP's managed datastores because I really don't want to maintain my own database server (even if it's a managed service that someone else upgrades for me). So I've stuck to Google Cloud Datastore / Firestore, but I've been experimenting a lot with Litestream[0], and I think that might be my go-to choice in the future instead of proprietary managed datastores.
Litestream continuously streams data from a SQLite database to an S3 backend. It means that you can design your app to use SQLite and then sync the database to any S3 provider. I designed a simple pastebin clone on top of Litestream, and I use it in production for my open source KVM over IP. It's worked great so far, though I'm admittedly putting a pretty gentle workload on it (a handful of requests per day).
Having worked with quite a few ex-Googlers this is a pretty standard Google engineering pattern.
True, I have to maintain state on S3, but there's not much work involved in that.
If I was maintaining my own database server, I have to manage upgrades, backups, and the complexity of running an additional server. With Litestream, I don't have to manage upgrades because nothing bad happens if I don't upgrade, whereas there are security risks running an unpatched MySQL/Postgres server in production. Litestream has built-in snapshots and can replicate to multiple S3 backends, so I'm not too worried about backups. And there's no server to maintain.
What operational complexity do you see in Litestream?
I don't see any operational complexity with Litestream.io. I think that's an awesome tool. But it's not that different of managing PostgreSQL backups with something like WAL-E.
The complexity of managing your own database server only exists if you don't use a managed service. Then there is no server to maintain and they do all the things you mentioned for you.
But it also introduces a few challenges. It's not as easy to connect to your database remotely to inspect it, with something like SequelPro for MySQL. It's not possible to create an index or drop a column without blocking all writes, which can be annoying if your database is large. Database migrations in general are harder with SQLite because ALTER TABLE is limited. [1]
One last thing regarding losing the few seconds of data. If you use something like Google Cloud Regional Persistent Disk, then your data are replicated synchronously in two different data centers, which means you can lose your server, restart another one, and not lose any data. Can still be combined with Litestream for backup to S3 with point-in-time restores.