MariaDB — I am familiar with the basics but only at a high level. Reasonable driver support. Some very nice features for the use case, however, something to look into later.
Firebase and other hosted solutions — would work well enough, but would be expensive to run at scale and most options here tie you to a specific cloud provider.
CockroachDB — Mostly seems like overkill and adds complexity.
Mongo — I've been around too long, the name is still a curse.