Honestly, I get really annoyed at historical shows that throw in ridiculous anachronistic bullshit just to try and check boxes and broaden their fan base. I love historical fiction but I'll turn a show off really fast if it starts getting way more fiction than history- especially if it feels like it's pushing an agenda.
That said, I haven't felt that way about Last Kingdom to this point (still on ep 2 of the most recent season). I was worried it was going that route when they made Æthelflæd ruler of Mercia, because I thought something like that would have been extremely unlikely for the time.
I looked it up and apparently it actually happened pretty much the way it was portrayed. Yes, it WAS an extremely rare and unusual event, but it actually happened.
From what I'm gathering, it wasn't completely unknown for "Viking" cultures to have "women warriors" like Brida. Rare? Yes. But not unheard of.
So I don't think it's really that unreasonable to imagine that someone like she could have existed. A little stretch that she would be that "badass"? Maybe, but definitely not enough for me to ditch the whole series.