So my bicep is growing, but I'm not developing a peak.

I've tried preachers/cable curls/bent over curls etc. Still no results after 4+ months.
Could it just be shit genetics because my father has thick arms just no peak.

Time to hop on some gear. That'll definitely tell you if its genetics or not.

Long, thick biceps > peak imo

It's genetics based on muscle insertion

Do you also try partial movements?

Instead of a standing barbell curl with full range of motion, you should try not going any deeper than horizontally and go back up for the peak contraction instead of the full stretch.

I always credit my isolation curls to really add to the peak as well as squeezing the biceps at the top of these exercises.