During university, we've built OptiTravel (https://github.com/denysvitali/optitravel) to do something similar. We couldn't use Google Maps APIs (project requirement), so we wrote a custom routing algorithm based on A* and I've created a Rust server to host GTFS data (https://github.com/denysvitali/gtfs-server) à la Transitland (https://transit.land/).

Performance wasn't great since everything had to run locally and do network roundtrips, but it found routes in my hometown that Google Maps didn't show. Pretty cool discovering hidden connections in the transit network and being able to customize your own params (https://github.com/denysvitali/optitravel/blob/master/src/ma...)