You can self host cockroach db which scales horizontally well. But I think going "fire and forget" while also having this much concern about "scaling up" on some level feels at odds with each other. Lots of DBs can just scale up vertically or horizontally pretty easily, but as it gets bigger you will want to know more and more about how to administer it properly. As you scale, risk gets bigger. So either "fire and forget", don't learn much, and cross your fingers. Or, plan for the future and plan on getting good at some database.