What Jack needs is lots of time & energy.
1. Get the game running smoothly, never worry about updates and tweaks.
2. Get your app and game migration to HTML5 & WebGL technology up and running.
3. Updates & tweaks. Hire a design team for each race type or simply do away with 25% and 75% racing.
reason - each race type needs its own engine & tyre wear rates and track evolution rates to get the best gaming experience out of them.
That's where the design team for each race type comes into it. Because there will be lots of code re-writing and editing for each race type & that's not one team job.
You already have a great game you just need few wee tweaks here and there to improve it.
4. Sit back and reap your rewards. Play a few games, think up some more ideas and see where things can be improved more.