Try the EventLab AFK method. Try the auction house. If you still feel the need for a trainer, at least back up your clean save file and disconnect from the internet.
A surprising number of "legit" players use trainers only for changes: Forza Horizon 5 Pc Trainer
Trainers/Cheats for FH5...? - Official Forza Community Forums Try the EventLab AFK method
No trainer is 100% safe. Even private, paid trainers have been cracked and signatures sent to Microsoft. Forza Horizon 5 Pc Trainer
The holy grail. Many trainers offer "Add 10,000,000 Credits" or "Unlimited Wheelspins." This effectively removes the entire progression economy. Want a Shelby Daytona? The 30-million-credit price tag becomes irrelevant.