Don't fix things if they aren't broke.
Still need to place the pas sensor (trying to come up with a bracket)
The "ring" that came w/ your PAS sensor is malleable and can be shaped to fit the round contour of the underside of your bottom bracket and epoxied there. Usually the sensor needs to be turned around(Use the existing screw holes). W/ the bike upside down, hold the bracket close to the disc and test by cranking the bike by hand. The sensor needs to be close to the disc. If you don't want to pull the crank to install the disc, Ebike CA sells a 2-piece disc that snaps together;
1. I got the bike up to around 40km/h, is this normal for a 29” q128c 48v (201 rpm 36v motor) ?
Feels a little slow? I Might just be getting use to it.
Yeah, you should have gone w/. the "260" motor.
2. I have schwalbe cruiser road tires, hardly used, brand new tread, should I be buying specific ebike tires ?
Some of us go w/ wider tires for stability, which requires wider rims, but at your top speed, it's not necessary. If you start to experience flats, you might consider the flat-resistant tires. I like the Forte Gothams and the Forte extra thick tubes.
3. I have 2 torque arms, not a lot of space on the axel to place the 2nd arm, is one fine ?
I perfer two pr.s of TA's for piece of mind. One side is usually easy while the other can be tricky. Sometimes the axle piece of the genaric TA can be used and a "link" piece that bolts to the brk. caliper can be fab'ed;
On my other bike, I used this special shaped TA from Ebike CA(Grin);
4. Got some small reed sensors on the way for my existing brakes.
I epoxy mine to the lever bracket and stick a sm. magnet on the lever w/ double-sided sticky tape;
should I buy bigger brakes? Avid/sram db1 brakes with 160mm rotors
Can you stop the bike? Both of my bikes have 160 mm brk.s and I weigh 250 lb.s and have no problem stopping on a dime. If you don't know if your brk.s are strong enough or not when riding, then I'm guessing you don't have much riding experience. You should practice hard stops in a parking lot. A good rider can actually do "brakies", lifting the rear tire off the ground, although it's not something you actually want to do in a panic stop. Only after you are good on the brk.s and they don't feel stroung enough, should you think about larger ones.
EBC makes Ebike brk. pads that last a little longer.
Was going to shorten the cables or do people just hide them in a bag?
People do both. I shorten my cables which requires soldering skills, a bunch of connectors, etc. And after I shorten them, I hide them in a bag. Where is your batt. now? Your bike has a big open frame w/ lot's of space, You might as well use it and install one of the frame bag types.
its a GT Karakoram MTB, has lock out forks, but not exactly the lightest bike I dont think...
Weight on an ebike doesn't matter much unless you are carrying it up and down stairs.