I have a version of the 15 tl. I think most people have a version 15 td. Given that to most people here sd card works, worth would try to install this newer version 15 td?
I unfortunately still far less expert than you. I dread that I make it brick when try to reinstall. Incidentally, in my tablet does not work properly USB3 (slow) sd card (like you), tablet is shuts down instead of put to sleep, It also spontaneously restarts over and over in android. Now I know that the Chinese tablets is only for those who have it can then finish to a usable state. 🙂