Trouble fitting new crankset – compatibility issues

Hi there,

I'll try to sum things up :

• I have a 2014 Trek DS 8.6 that needed some TLC. I'm a wrenching newbie but I don't mind the struggle.
• I installed a brand new Shimano Cues groupset with great success (FC-U6000-1 crankset, RD-U6000 derailleur, CS-LG700-11 cassette, MT501 bottom bracket). The bike rides smoothly.
• Only issue : I realized a 32 teeth crankset isn't enough, I want a 42 teeth one at least.
• I got a new 42 teeth chainring (U6000), but I learned the hard way what PCD is. It's not compatible with my crankset (96mm Vs 110mm PCD).
• I then got a whole new crankset, a Shimano Cues FC-U6030-1. Comes with a 42 teeth chainring.
• I installed said crankset without any issue, until I tried to install the plastic cap on the crankset. It barely holds, looks like the spindle is too short (?).

I can't figure out what I missed. I tried to ride the bike anyway, but I lost a crank arm after a couple minutes (bike felt great though). I guess it's a bad sign.

What would be the most clever move from there ? Is there something I missed installing the new crankset ? Can I use one of cranksets somehow ? I was recommended a U8000 chainring (after ordering the second crankset sadly). Since it's pretty pricey (specially after messing up 3 times) I'd like to be sure it's the best solution with the parts I now have.