I found that even a proprietary product can improve your skills in a general way, and it has the advantage of being pre-selected for "what works". Anecdata: Google Firebase, specifically Firestore. On a superficial level, it's just another NoSQL database and you'd probably (rightfully) realize all the things that MongoDB can do that Firebase cannot. OTOH, I learned two things from it that I didn't from previous work with MySQL and PostgreSQL. (Note that I did not previously work with MongoDB beyond some tinkering, so maybe that would work as well).

The first was data-change notifications, i.e. getting notified when a record changes. I toyed around with this idea with MySQL some time ago, masquerading my server as a replica server, but generating derived data form it. IIRC PostgreSQL has something similar with WAL streaming. With Firebase, I learned how this can actually work and convinced me that the idea is sound. It definitely encourages me to actually do this for real when I work with PostgreSQL again.

The second is what PostgreSQL calls row-level security, i.e. using the data in a row to determine required permissions. In Firestore, this is the only kind of permission control (except for all-in admin accounts), and it showed me how this easily prevents a whole range of security issues that can occur through bugs in a back-end server.

The specific coding around Firestore is shallow as you said, and non-transferable, but understanding what is possible is something that will definitely help me in the future, with other databases.