Invert conditioning : Chin ups / Pull ups ?

I’m still struggling to pull my ass up in inverts. I don’t know if it’s a core problem or lats / arm problem. My back is rounded and I always end up horizontal rather than vertical. So in the case, to hook my leg in outside leg hang I always have to scoot up, this makes my flow less flowy and natural. I want to fix it. I condition a lot outside of pole.

However I’m wondering if Pull ups (in neutral grip) or chin ups are better to work on this ?

Also any good core exercice to be able to lift my legs ? I can lift them but keeping my butt over my head seems impossible !

Thanks :)