For this purpose, I’ll sometimes modify a workout by doing higher power intervals at a low cadence, sometimes standing, instead of the instructed cadence. On longer workouts, it also gives my butt a rest.
Also tbh I think strength work on the bike will be more efficient than strength work in the gym, if the goal is to push more watts. If you focus on pure strength in the gym (e.g. low reps, high effort, long recovery) you are working anaerobically. That’s not going to help you much when you want to push more watts on an extremely steep climb for a couple of minutes.
I think (but that’s pure speculation, maybe some coaches can chime in) that you’re better off doing hypertrophy work in the gym (mass building in the legs) and complement this with strength training on the bike (aka low cadence work) as the hypertrophy work will give you bigger muscle fibers to work with, and the bike work will learn your body to use those extra fibers more efficiently in longer durations.
Purely doing longer duration squats (20-30 reps) still is nothing compared to a multi-minute steep climb on the bike. I have a hard time imaging how those 20 reps is going to help you push for 4 minutes on the bike as your squat training set will take 1 minute max.
I don’t want to say strength work is useless for general conditioning and health, but I don’t think it translates to better performance on the bike as much as some might think.