I guess bumping your own topic is okay in this situation and a new one would be redundant.
I have to carefully read the complete thread again after this long time so this reply might be incomplete and I might edit it later. My main statement still stands: I suspect the custom firmware has been uninstalled and non-legit titles prevent System Settings from working. A custom keyboard will also not work when CFW is deleted. The ntrboot method is the main unbricking method if there is no B9S installed.
Since there are some new exploits (browser based) you might have luck if you are on latest version (or can update to latest version via safe mode). Note that the 3DS has to be connected to the internet and the non working keyboard makes trouble. See if you can successfully complete one of those online methods:
Offline methods:
https://3ds.hacks.guide/homebrew-launcher-(soundhax) (Official firmware must be 11.3 or lower)
and... You updated in 2019 according to this thread so Soundhax is out.
ntrboot; any firmware version, offline, even if some parts of the main operating system are missing, damaged or not working.
https://3ds.hacks.guide/installing-boot9strap-(ntrboot)