I don't think the leaf spring mock-up looks bad, just not necessarily great either. I think it would be tits if you could run a torsion bar where the pivot is now between the swing arm and the frame. Use something like a hex or octagonal torsion bar(whatever you would need to make it work, these are just what came to mind), have the appropriate female shape in the frame and on the ends of the swing arm but leaving some room inbetween the two for flex. Something holding it tight/still in the center(the frame), some space for movement, then something holding the ends(the swing arm). That would be my #1 choice for suspension. Otherwise, could a mono or dual shock set-up work where the bottom of the shocks attach to the brace between the two sides of the swing arm and then the tops would mount to a mount that would sit right under the seat? You could hard mount the seat to this mount and weld the mount to the frame so it is actually a hard mount but looks like the springs are holding up the seat.
Just spitballing ideas here, feel free to disregard me, lol.
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