Heya,
The reason Mason is still the route in development is simply making assets, improving the code, budgeting, and recovering after some bumps in the road. After Mason's route is complete, development will be much faster since most of the main character sprites will be complete. I'm doing it route-by-route simply for consistency. His route will end in 2027 and Kelsey's will hopefully start in the Fall.
Many of us also have outside work of Silverstone, which factors into that. Regardless, Mason's route will be ending sometime in 2027. Hopefully in the summer, but we shall see.
I know a lot of people want to get to the other characters, but it will be worth the wait!