Usually the lower limit screw adjustment prevents this, but I’m sure you’ve tried tweaking that. I’m surprised this is happening while you’re shifting up, the derailleur is moving in the wrong direction, very confusing.
The star nut + cap bolt are just to preload, they should not hold anything together while you’re riding. The bottom part of the boom clamp, where attached to the steerer tube, is what should be holding the headset down. Make sure that’s tightened, and that you have the proper spacers on the...
It does look like your problem would be solved by a shorter chainstay.
Or a curved slider, or a steerer tube extender.
Multiple ways you could go here.
I am contemplating the sale of my beloved Silvio 2.0, since I am about to relocate internationally and I cannot easily use or store it at my destination. My plan has been to store the bike with family in Massachusetts, but that seems like a waste of a good bike (plus I wouldn't mind upgrading to...