IME and opinion the better choice is to strengthen your back rather than change your fit. Back hurting on the bike is a symptom of a problem. The OP is pretty young to start giving up. I stretch, plank, and do pushups every morning plus go to the gym for an hour twice a week. And ride my bike. My morning little workout plus the gym is the minimum I can do and still be able to ride at 78. I joined my first gym in '79 and have been going ever since. Once I could do all the exercises in good form, I started increasing the weights to where I'll just barely be able to complete the 3rd set of 10 or 12 reps. Light weights don't do the same thing. Almost every exercise in the gym works my back in some way. One's back is always supporting something.
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