Postgres can also run on the Pi.

More than once I've started a project with sqlite and then had to migrate to postgres. In my experience it's because of the way sqlite has to lock the whole database file while writing to it, blocking concurrent reads - this isn't a problem in postgres. (There's WAL mode, but it still serialises all writes, and requires periodic explicit checkpointing IME)

You may also find you really want a feature postgres has, for example more advanced data types (arrays, JSON), more advanced indices (GIN inverted index on array members), replication...