> None of those are digraphs or have diacritics, each is a single letter/character

Okay, you got me, these don't have diacritics, but other Slavic languages do. Unicode committee decided that some of these are separate letters (Ѓ, Й, Ё etc.) and some are not (Ў), but still doesn't make these "trivially provable to suit Slavic languages better".

> Nj (this "letter" is made up of two other letters)

Indeed it is. Invented in 1818. E.g. Russian uses two "нь" for the same thing.

My point is, even if I may confuse my linguistic terminology from time to time, is... How does all this make Cyrillic "trivially provable" to be better suited for Slavic languages than Latin script? It's all the same: invent new letters or new letter combinations, or slap a few diacritics on top. And when that is not enough, borrow from Latin. E.g., j in Serbian, ї in Ukrainian and й in Russian for the same sound.