You need the right switch, two di2 wires (from shifter to display and from motor to derailleur) and the derailleur. Change to di2 shifting with stunlocker, done.
Z1 is basically a rebadged 36 so it should be quite better. 12 speed is a bit more work to keep shifting smoothly, I see it comes with the Deore shifter which I would replace with XT (multi upshifts, faster) Tires are quite a bit better on the 700.
The 35 is truly a entry level fork but other...
That's not a bug, it's by design. There has to be 0 load on the torque sensor as it initializes so it knows it's starting value. If not, the power delivery would not be accurate when you would load the pedals.
Anyone running riser bars? Are the stock bars considered flat or a slight rise? (if I go 20mm rise I doubt it will be noticable so I'd probably need 35 mm)
There shouldn't be much difference between 2.5 and 2.8 plus? I also have 2.5 Assegai on front and 2.6 Martello rear (which I assume are "regular" 27.5 tires) and think there's enough clearance.