You should have titled the thread "Database settings in the main Kodi GUI" (leaving out the "external"), because all the DBs are external to the Kodi app, really. The word "external" makes it sound like you're asking for something far removed, when in fact - you are not. Perhaps "alternate database" would be more appropriate. Just because its on another box shouldn't make it a second-class citizen.
So I have a use-case for y'all where this could be helpful. I just deployed Kodi onto a non-jailbroken iPad. Works great as a simple client. However, I have a centralized MariaDB setup with multiple Kodi clients, and as this IOS device is non-jailbroken, there is no easy way to edit the config files like advancedsettings.xml to set up the DB or otherwise. Granted, this is a "non-standard" deployment. And, I can understand the idea that "most people" won't use a centralized DB AND that there's a desire to KISS. However, I could rattle off any number of advanced settings
already in the current GUI menu that "most people" won't use. Having the various menu levels which expose things which are considered more advanced only at certain levels seems to have been put in place just for this type of advanced configuration. So, it does strike me as a bit odd that, after all these years, you still have to do this outside of the GUI interface. Trust me I am as much a command-line guy as any, but just as I rarely use an IDE to debug code - its still nice to have it available in certain situations. Just sayin
As an addendum, I would also add that there
are already direct GUI DB config solutions for OSMC and LibreElec which I also have found to be useful at times. They to do it through an add-on, but I reckon they have altered the Kodi source to make this possible in their builds.