Add a rollcage.
Get the weight as low as possible.
Ad tuneable suspension, ARB’s and differential.
Increase tyre width.
Then maybe add better tyres if it’s appropriate to the car level.
If not, start adding horsepower and grip in balance, with a preference to grip.
(Also. Rather dissapointed that you can’t take the racing truck into online lobbies. I’d had great fun with the Hummer in FM4 winning races with it and wanted to do the same thing)