i forgot to mention pulleys. No offense to Enertion, but i’m not a fan of the 3d printed wheel pulley. Especially not for bigger riders like you and I. Too much torque and faster wear. I had one printed in the strongest Nylon on Shapeways about a year ago. I have it on my GF’s board and even when tensioned as tight as i’m comfortable - it will still click on hard braking in particular. That means skipping.
Now maybe he’s using a new uber material, and perhaps some more aggressive teeth profile to grab the belt? The 12mm (vs mine at 9mm) absolutely will help! BUT at the end of the day it’s going to wear faster with us bigger guys. I would spend a few $ more and get one in aluminum or steel. I really like the older kits he had with the nice anodized blue aluminum wheel gears - but doesn’t look like it’s offered any longer. bummer. My current favorite is the DIYes one which is aluminum and also hollowed out under the gear/teeth which gives a smidge more clearance around the axle of the truck. Minor feature and not sure if he still has these…
And again 15mm wide belt and gears for a single setup - more contact area will help when properly tensioned to keep it from skipping when braking hard. Dual 9mm has been fine for me in the past, but if i can fit 12 or 15mm i might do that just to have more contact area.