Yes, I think a shortening of the second nap, a means to phase it out, might help. Usually in this case, you want to preserve the first nap at whatever length he'll take... so don't cap that one bc it could cause OT if we did. We're trying to balance a few things- total daytime sleep, awake times between sleep times, and total hrs awake, all in order to get decent overnight sleep. Lots of trial and error to get the perfect recipe as you are aware!
Not sure when you started the pm nap in any of these instances, but a this age and stage, I would only try the pm nap after 4.5 hrs or so. Then BT 3-3.5 hrs after.
Something like:
WU 5:45/6:00 (hopefully!)
NAP1 9:45-11
NAP2 3:30-4:00
BT 7:30