This is not really a guide, but more a hint if someone is getting stuck with the achievement Hot Wheels Legend in Forza Horizon 5 not unlocking.
If you have won the race and meet al the requirements to unlock the Legend rank, and still haven't gotten the unlock then check if there is a seasonal event running.
When you start the race, in the menu where you select if you want to play Solo, Co-op or PVP, look at the bottom and you will see the "Switch to Normal Event" button.
The win during a seasonal event won't count towards this achievement, you will need to switch to the normal event.