You know for once i'd like to see the FL choose some other guy in a situation like this. I...

Same

I would have took the red haired guy and left already

There's no time to be picky
A holy knight is enough right?

Yeah. They main character never has another way to become stronger, than to MARRY. There are some ways you can do that, without marrying?

Exactly. She first came to the temple looking for a holy knight before she met the ML. The red hair is a commander and the brown hair guy, the one always beside the ML is a general or something. They're both good husband candidate with good position and background and they would be more willing (compared to the ML) if mc proposed her conditions to them instead. According to the situation, all she needs is to find a better husband candidate than the guy her sister is marrying. She says she doesn't have enough time and yet here she is, wasting time on the ML who has already rejected her and even thrown her out.
I honestly don't get these FL leads sometimes.

Yes this. Its VERY rare to see a FL grow stronger without having to depend on the ML. Its like no matter how rich, strong or beautiful the FL is, the ML is somehow always more powerful, stronger, richer and more good looking.

I feel like it's a sad reality, you know? People at those times were pretty annoying with the "women who aren't married after 18 is bad luck" thing... atleast her father is willing to make her an heir even though he's a little materialistic he still doesn't mind making her the head unless there's someone more competent than her

I agree with everything you say, but just to clarify, the reason why she's insisting to get Killian as her husband is because she's thinking what her father would think. She thinks he might be disappointed in her if he knew a man of Killian's caliber is present in the temple but she didn't manage to 'get' him.