Magura MT5's are pretty much the best ebike brakes around. You can go crazy with say Hope brakes or something, so its possible to spend a lot more and get a little better stopping power.
Best option is the MT5 brakes and then buy the cheaper, and better, Tektro Type 17 2.3mm thick rotors. They were originally a niche product for downhill bikes but then Tektro realized they could call them ebike rotors and sell them by the shipload. MT5's normally use proprietary, thicker 2,0mm rotors and the 2.3's work, albeit just barely when the pads and rotors are both fresh/new.
tektro.trpcycling.com
The site says $32 each rotor but you can find them online for $25.
The MT5e is an ebike-capable brake with much improved brake levers, which are meant to satisfy certain moped ebike regulations in the EU but which end up just being a lot better. They use red HIGO cutoff plugs, which you can either buy adapters for, or just buy a compatible wiring harness that has red plugs instead of yellow ones. California Ebike sells those.
This e-bus cable is the main wiring harness which extends from the motor to the handlebars and allows you to connect throttle, brakes and display. This cable is required when using the Magura MT5e eBike Disc Brake It features two Higo 2 conductors (Red) plugs which are compatible with Magura...
california-ebike.com
Here are the red-to-yellow adapters. Its cheaper just to buy a replacement wiring harness, and cleaner too.
HIGO-A-BB-B2-S3 Mini-B adapter for Magura MT4e and MT5e to connect to Bafang BBS01, BBS02 and BBSHD
www.e-bike-technologies.de