Books with old women as anti-heroes

"Marco and the Red Granny" by Mur Lafferty springs to mind. IIRC, she ended up releasing it as a free audio book.

In Scott Lynch's Red Seas Under Red Skies, there's a kick ass older woman pirate, and I think he's done a fantastic job with her. She's actually a cool character rather than being stereotyped in any way.

It's really cool that you're doing something different with your protag btw! I've gone the anti hero route too and it's lots of fun