There is an option to do MySQL on a LE client. See LibreELEC .
I'm not yet sure where your media collection is stored. if it is also on the same headless Kodi device, then you will need to start using pathsubstitution on all other Kodi devices. Kodi stores the full path of the video/music track, so that won't work out of the box with other Kodi clients. Once all your Kodi clients are set up for use of your headless Kodi box, any of them can add new items to your video and/or music library, because of the shared database. Each Kodi client will download fanart and thumbs and such instantly once its local thumb cache is checked.
Personally I wouldn't run MySQL off an SD card. It's too flakey as a safe storage solution. There is no TRIM support, and wear & tear will sooner or later corrupt your data.