Choosing Self-hosted Database for Mobile Application?

Question

API

Task:
there are many clients and their data that do not intersect in any way, i.e. each client
has its own data server with a database, and some api
It seems that mysql and the like for storing such data is not very optimal, and will create an unnecessary load when fetching data.
DataSync from Yandex is not very convenient and requires authorization from Yandex.
realm.io in the free version does not provide access to data, and the paid version starts at $1.5k, which is for a small non-profit project, somehow does not work.
What would you recommend to use and why?
UPD: Among other similar services, I did not find a suitable one for myself, so I tend to raise something on my server.

Answer 1

Firebase for example. Previously, parse was used for this, but now they have moved to firebase