Pricing aside (as it's almost unreasonably expansive if your app requires frequent db writes), firestore is indeed "A magical database that has perfect automatic horizontal scaling". But as you have your happy setup on aws it probably makes little sense to switch.
Yeah there are a few in that category also Google Cloud Spanner and the stuff by CitusData. All of them work, but are prohibitively expensive to get started. I've harangued them a number of times about how people aren't going to want to use something they can't scale up from $1/month to $10000/month without migrating any data (that's the whole point of an auto-scaling horizontal service imo), but so far no changes from them. Like why would you design potentially infinite auto-scaling, and then lock it up behind a $90/month minimum fee. They could have been making money all along on those $20/month or $40/month or $5/monthers, who vastly outnumber those who _need_ autoscaling, but what the peace of mind that auto-scaling provides.
Hmm I wonder if spanner has more minimum hardware costs or something. Like if they have to provision you at least one standalone atomic clock to get started.
But like that's sort of my point. They could have data default to going into a tiny VPS slice that would be free-tier territory, and then automatically move it to whatever infrastructure spanner requires when the time comes. That could all be seamless. Why keep the seams in?
$90/month is really not that much, it's like an m5.large on AWS which is pretty much the minimum for a medium/high traffic website. Well, I guess the scene might have changed since they introduced t2 unlimited, but that would only take it down to $40/month. Support costs are still there for a $5/month user.
