How do i stop the overeving when changing gear with my steering wheel buttons?

By lifting your right foot off the throttle momentarily.

there used to be a setting to stop it, has it gone?

Yes, it's gone.

Yup, it was an unrealistic setting on the sort of cars LFS features.

Flatshifting (what you're doing) is slighty faster AFAIK but you'll burn out your clutch doing it.