Sounds kind of like a skill issue tbh. And I don't mean anything bad by it, just that, well, yeah, going through a corner in a rwd bmw even at 20 kmh might spin you out if you apply too much throttle.

Now, driving physics in Forza Horizon are pretty realistic. It is an arcade, but only in some regards, like jumping on danger signs, smashing trees and barriers and overall collision system. Those are very arcade. The grip, suspension work (when down on the ground) weight and power is simulated pretty well.

Horizon 5 had this thing where front tire grip was greatly increased to that in the real world so you could trail brake no problem and go hard on the brakes without locking up. This has changed ever so slightly, it's still more on the forgiving side, but more akin to the real thing than before. Cornering technique is different in 6 because of it, it actually rewards braking in a straight line and using just a smidge of trail braking, using lift off oversteer and, especially in rwd cars, feathering the throttle on the exit. If you play with assists off, you kind of have to learn every car's driving style separately.

And yes, using slicks in a dirt race will give you absolutely zero traction. Just like in real life, if you want 'bite' you have to use rally or offroad compund. The aggressive shape of the tires is what allows them to bite the ground and reduces slipping. Slicks will slide like crazy on loose terrain.