I don’t use any of them, and they represent a bulky disk and RAM usage, just idling. Also concerned that MariaDB and other “helper” apps aren’t true FOSS. Preferably, I would like to find a way to remove them declaratively in NixOS. I have posted on Nixos discourse, but so far haven’t received any useable input.
MariaDB is FOSS but MySQL is the problem one as I understand it.
Not a direct answer to your question, but as of late SQLite is the default DB for Akonadi. Maybe that eleviates some of your concern.