The AL frame bikes I’ve had were very harsh rides. But one of my carbon bikes was really stiff but not quite as harsh.
For me the 3 big things for back and overall comfort are bike fit, frame material and design then wheels/tires. Ideally your wheels would be a lightweight carbon with wide enough width to accommodate wider tires.
Be sure to find a fitter who can adjust your fit to meet your needs versus someone who just fits everyone only using one method.
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Ride your Ride!!