I even tried to use the official TES-CS, but it does not allow one to work at the master file. but no posts of such files anywhere are found, which is unfortunate.

I`ve been reading that some people, on other forums/sites, were wishing for the same, and maybe they came up with a solution or a custom level list. Face them, or never level up to level 16-18. It seems I and others who are affraid of these critters have only two options then. I tried it, but the resulting plug-in file had errors in some strings, so, I guess, I can`t rely on it. You'll have to read the documentation to get the best of it though.

It has an option to block spawning using keyword filters. This one might help you - Dynamic Leveled Lists by Leviathan1753.