I have my own Nextcloud instance which uses PostgreSQL to store its data. I have nothing against PostgreSQL, but it seems to be an overkill for a single-server deployment used by a single user. Well, it’s used occasionally by other people but the number of concurrent users is always low.

Nextcloud documentation states that it doesn’t care much about your choice of a database, and you’re free to use SQLite. In fact. SQLite is a default data backend for a Nextcloud instances. The problem is: the documentation also advises against using SQLite for anything but minimal deployments, but it’s hard to say what it means by minimal. It might be concerned about scalability, which is not really an issue in my case, but it could also mean correctness or stability. I guess the only way to find out if it’s OK to use SQLite is to try it out.