More weight I.e larger legs will slow you. You want stronger legs, fitter legs and less weight. Pro road riders have something like 5w power per kg. Say cat 3 riders about 3wkg.
Thin legs you’ve developed by long endurance rides. Easy rides up the hill. To gain strength I’d suggest short 45 min rides doing hill reps a max heart rate.
Do you mix up rides? Short fast? Long slow? Got a turbo?
My solution is a zero cost option to, just use your current bike