I (and Claude) have been working on an open source monte carlo based retirement tool for quite a while now. Its definitely geared towards people with more individual financial ownership and tech knowledge but I feel it gives good flexibility for me to model my situation. I had initially gravitated to this project because I think Monte Carlo and numerical methods are just so dang fun.
https://github.com/jgrazian/finplan
Try it live: https://finplan.rayknot.com