That does look extremely fiddly, especially with not much cable to work with. I'd be inclined to just extend the coax down to the hub and skip the junction box. Can always insert one later, and you'll have more length to work with.
Edit: I may have misunderstood the Son light wiring completely, I've soldered Son coax before but mated it to a B+M light via crimped spade connectors at the light end.
https://www.peterwhitecycles.com/downloads/sch-y-junc-inst.pdf
That does look extremely fiddly, especially with not much cable to work with. I'd be inclined to just extend the coax down to the hub and skip the junction box. Can always insert one later, and you'll have more length to work with.
Edit: I may have misunderstood the Son light wiring completely, I've soldered Son coax before but mated it to a B+M light via crimped spade connectors at the light end.