OP; Although the "geometry is spot on" I don't see where that bike, as it is presented in the photo, really fits you very well.
The seat seems to be way back to the extreme, the frame seems several inches too small (seat tube) and could benefit from conversion to flat bars and a longer stem, etc., followed by adjusting the seat position. Probably just moving all the components over to a larger frame would be a big improvement. Tire size and pressure could be a cause of some issues. Likewise, adding a rack to allow you to get that backpack weight off your shoulders/spine/butt would really feel better.
Re the frame material issue; I don't see that it can be really labelled the bag element here. Too much else to fix first. It is true that some AL bikes can be harsh rides under some circumstances (as per Frank's input), many aren't. Same for some steel frames. I have heard more folks complain about C'dales being harsh (due to large tubes) than others but it is still not that common of a complaint.
Re a recumbent; A good, well fitted recliner bike (probably not the one pictured) will be highly likely to be very comfy ride. They are a different sort of ride though that a classic design bike. If they work for you I would recommend them. You would want to borrow or rent one to get several good trips on it before investing. Initial impressions with them can be quite different from a more expanded use. Peoples reactions vary (a lot). You may not like them in the end or be totally in love.
FWIW;
/K