One thing I've noticed about RGRE fics is that 90-99% of them always have a human male as the protag, but one thing I've always been curious about is how an RGRE/matriarchy/herd fic would go if it was a human female involved. What would her reaction be? Will she actually take the new rules well? How would she compare to her usual male counterparts?

I'm either wishing for a fic like this or asking if there is such a thing.

The only RGRE stories that involve females (that I've seen) are some 'what-if' scenarios of existing stories.

Uh-hmmm's done a few.
https://www.fimfiction.net/story/494175/one-step-at-a-time
https://www.fimfiction.net/story/462117/pizza-pie-something-something-amore
https://www.fimfiction.net/story/476892/knife-ears (this one's debatable)
https://pastebin.com/L10nmqRk
by other authors
https://pastebin.com/6iw6zGr2
https://ponepaste.org/1008

The story 'The long and short' might fit your needs. It's an anon story turned pony in RGRE but with plenty of pov chapters of the mares he interacts with.

7754441
Well, the thing is most Bronies are... Well, guys. So they wouldn't be able to reliably portray a woman's reaction to an RGR society.

That's the simple reason for the lack of coverage in that area.

7754532
That is indeed a reason.
We could write a story with a female human, but as you say, our options would be limited. In my case, the character will certainly lack depth.

Another mares treasure Best i could get. Hopefully this is kinda what you're looking for? Idk. I've read a lot of rgr stuff and human women from non-rgr in rgre is pretty rare.

If you're ok with non-pony related rgr then the teisou gyakuten sekai manga might also work for you since most of the chapters are from a woman's perspective.

Found a dam good one with a female lead for you.

Literally just stumbled on it today.

Synopsis: A foreign female warhorse finds herself in the Equestrian military. How will a blooded warhorse fit in?

Can be found here: https://ponepaste.org/7791