I'm currently going through the novels of this on amazon. (I'm on vol 8 of the 11 listed, with release apparently ongoing.)
I love this series! I'm currently on vol 8 of the translated novel via Amazon/Kindle.
Feels like it's written for adults rather than teenagers. MC is about 25 yo, and pretty much acts that way. Genius, but decent guy (girl now). Has character growth appropriate to that age. MC's time is split between teaching (with focus on outstanding, future hero/heroine students), and various massive incidents. While MC is extremely good, he's not "blessed" by the world (stars). Many of her battles are very difficult without easy wins. And she's already paid massive price for wins (and escapes from disasters) in the past.
Quitting/Kicked from the party... Avoids all the usual tropes. MC is not some doormat loser (quite the opposite). And the hero party, while suffering from personality problems, is legitimately competent. The cause of the quit/kick is more complicated than it first appears. Also, MC was known and well respected. (Sure many people focused on the Hero, but MC had plenty of stans as well.)
Demon Lord: It's complicated. Background lore takes a long time to develop, but story does get there, and it is important.
Gender Swap: Yeah, the story could be pretty much the same without it. Main relevance to plot is that very few people learn that the MC "Rania" is the same person as the previous male "Raniel". But the swap is also somewhat emblematic of Raniel/Rania's character growth -- so it's a good use of the GS trope.
As for the manga version... It's what I started on. It's not bad, and I enjoyed the visuals, but... it feels like it's pitched at teenagers. Though of course, manga necessarily present simplified versions of text novels, so that's not entirely surprising.