I have read through the other responses and agree with what has been said here. I'd like to add my philosophy of the 3 S's. Stability, Specificity and Speed. In that order. You want to start from a stable base of support, glutes, abs and core in general. Then add movements that are specific to cycling like step-ups and lunges and only add speed once you've gained good control in the first two categories.
Here's an example. Do a squat with a depth that simulates the range of motion required through the pedal stroke, for many that would have the thigh at horizontal at the bottom of the squat. If you can't hold your weight balanced on your heels and keep your knees turned out, you don't yet have "Stability" and need to work on that first, once the movement is stable (with no collapsing in of the knees) then you can switch to lunges or step ups which are more specific to cycling because it is one leg at a time. When you can do those exercises with control, then you add some speed to the movement to generate power.
If you skip the stabilty and specificity steps you can set yourself up for injury in the speed phase.
And of course, time in the saddle is as specific as it gets.
Good luck and have fun.
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