Odd coincidence that this thread got revived today.
Sorry, can’t answer your Di2 junction box question because I don’t know the answer.
However, I entered my first 3/4 race this weekend and had my ass handed to me. My only goal was to survive and I barely did that. Ended up back of the pack in a crit, and then 44th out of 63 in a circuit race. I was one of the last riders to not DNF in both races.
I DNF’d my first three Cat 5 crits. Now I can hang in a 4 or 4/5 crit, and hold on for dear life in a 3/4 crit. I’m getting better, slowly. Remembering how demoralizing it was to get dropped on the second lap of a Cat 5 race, I have a lot of appreciation and respect for the sport. It’s immensely rewarding because it’s completely unforgiving. The barrier of entry is high and it’s unlike any other sport I’ve ever participated in, including MTB XC and DH.