[1] https://store.steampowered.com/app/255710/Cities_Skylines/
I was bothered by this problem so I solved it for my own city builder game, Archapolis (still very early development):
https://www.youtube.com/watch?v=7q0l87hwmkI
I can path 200,000 to 300,000+ units simultaneously to random destinations, with a bonus of utilizing all shortest paths between two points.
The paths are all cached for constant time path finding, but I figured out how to do so efficiently (both building and storing tree/graph)
I wrote a non-technical article here:
https://www.yesboxstudios.com/2022/04/27/all-nck-shortest-pa...
[1] https://store.steampowered.com/app/255710/Cities_Skylines/