Some guys on Tririg (not XTR but some good internal setups) and Weightweenies (some nice XTR Di2 CX builds without using the “STI’s” for shifting but a climbing satellite shifter instead) often use one of the original, much smaller Junction A boxes with the splitter box if more connections are needed. It seems so long as you have a shifter with a PCB (i.e. not the sprint shifters) then you can mix and match any ETube stuff however you like.
You can drop the Junction box into the downtube (when the fork is removed- cable comes out through drilled out front mech cable ports) if there is enough gap to give access for charging around the BB (99% sure my frame should be fine as the gap is quite big). There are also SLS junction box mounts replacing a 5mm headset spacer if you want to keep it out in the open but can’t be doing with the straps on the stem. Under a Garmin out front mount is another option.
The Pro Tharsis stuff looks nice on the face of it (Flow MTB hid a junction B box in the stem but kept the XTR Junction A for example) but it is designed around keeping the XTR Junction A and however you decide to use it you will always need to have the wire for the rear mech exiting the stem somewhere messy (at the headset top cap most likely). Plus the bar and stem aren’t the lightest 😆 I do like the idea of the battery in the fork steerer though like the Tharsis stuff lets you do and McConnell has done on his bike….
McConell’s bike is actually remarkably clean for something with so many wires! Shouldn’t be surprising as he is one of Shimano’s poster boys for XTR Di2