Nah, it’s a new, small community kinda gym, I actually have nagged them but because DBs over 15kg had fricking 5kg increments, but I always thought 2.5kg was fine for progressively overloading

2.5kg is fine for compound exercises, but smaller muscles don't grow stronger that quick, it's especially noticeable on movements you can't cheat to compensate.

Also I want to minimize branchioradialis involvement, they grow like weed and make my forearms look very disproportional

What sort of unicorn are you? Alongside calves they're the most stubborn muscles I got.

Stay away from pronated grip in that case, do curls in supinated position. Try out incline dumbbell curls or bayesian curls and see how those feel, personally I never feel it in my forearms on those, other than grip sometimes.