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That looks quite nice. I'm (slowly) building up a 650C Softride into a flat bar bike with XT Di2 and when I was picking out components the Pro Tharsis stem and handlebar combo also looked good to me. You can hide the junction box inside the stem, and it also has a collar underneath that you can use to preload the headset bearings, so that instead of a star nut you can put the Di2 battery inside the steerer (although I don't see why you can't just use an expander to preload the bearings, tighten the stem bolts, and then remove the expander to fit in the battery carrier). It also comes in road bike-suitable lengths and a 17-degree option. I didn't end up buying it because I'm in Canada and I couldn't find any shops that would ship it to me.
Fibreglass-
The problem I see with those is that I would firstly not want the junction hidden in the stem because it would mean removing the bars every time you need to charge and I’d look to use the bar end junction instead.
And secondly that it doesn’t give an easy option for routing wires into the frame, just in and out of the steerer for the battery
PhilDAS
Deda Vinci