Wen Spencer has "Tinker" series, which is a weird blend of soft science fiction and fantasy. I wouldn't call it exactly epic, but I like it, Alot, and it does end with a big war in Harbinger (comes out in 2022). And, more to the point, it has a large cast of well-written and diverse (as in "from different backgrounds and reacting to similar events in different ways", not as in "different skin colour") female characters, including Tinker herself. Some readers didn't like how Tinker took certain changes in her life in a stride, though. If you do get that feeling, wait until you read the spin-offs, where one female character goes through the same thing and doesn't handle it the same way. It's definitely YA for Tinker-focused novels though (still, there are many shorts that spin off the main novels, and they do have more adult-minded female protagonists).
"Mercy Thompson" series by Patricia Briggs is also not epic enough to be truly epic, but it does have ever-increasing stakes and scope. As the title suggests, it does have Mercedes "Mercy" Thompson as the protagonist, and is generally a good read. Another series by the same author, "Alpha & Omega" takes place in the same universe (AKA "Mercyverse") and has a male-female duo as the leading characters. Disclaimer: I like "Dresden Files" more than I like "Mercy Thompson". It's definitely not YA.
"Kitty Norville" series by Carrie Vaughn is, again, not quite epic (there's a series-spanning storyline about vampires and The Long Game, but it never reaches truly epic scale, even though the stakes (get it? Stakes. Vampires.) eventually get rather high), but it does have Kitty the Werewolf as the protagonist. Disclaimer: I like this series less than I like "Mercy Thompson". I would also not categorize it as YA. Not for easily-triggered female readers, since Kitty at the start of the series is rather weak, dependent and insecure.
"Charley Davidson" series by Darynda Jones starts as a mix of a mystery (Charley is a private detective) and gratuitous trashy porn, but gradually becomes more and more epic (although it always stays kind of local, in my opinion). Has a very female protagonist and lots of humor, although that humor often comes off as kind of low-brow. Absolutely not YA.
On the other end of the spectrum there's the "Wheel of Time" series by Robert Jordan (& Brandon Sanderson). It absolutely is epic, full stop (think "Lord of the Rings" kind of epic). But it also has multiple protagonists, with only some of them being female. Just deal with it.
Speaking of Brandon Sanderson, his "Mistborn" trilogy has a female character, Vin, as one of the most significant protagonists. I would also consider it to be very epic (because of the scope, not because it's a long series; "Mistborn" doesn't stop at 3 books, but the sequels take place hundreds of years later and involve completely different characters).
Christopher Nuttall has at least two fantasy series with female protagonists. Of these two I'm familiar only with "Schooled in Magic", and I can definitely say that it's epic, both in length and in scope (eventually, since it starts rather modestly). Though you might not like Emily, the protagonist, Alot, since The Word of God says she is not quite neuro-typical. This series would be best received by people who are into alternative history, because the author is an amateur historian and draws a lot of historical parallels between the events in the novels and in our history. Sadly (for you), it's kind of YA (with some caveats).
She's got her own code that she follows, and is disinclined to take crap from anyone. 