Let me help you a little with the brake issue. I am running a hydraulic disc on the front of the sidehack:
What you want is a scooter brake Master cylinder & calipers.
You do not want a larger master cylinder. Keeping the smaller master gives you more total braking force. Going larger would reduce available braking.
With the scooter front caliper in that picture, you get a pressure limiter. It is the cylinder hanging on the front of the caliper that the hose is attached to. The scooter folks call it "Anti-Lock". It is adjustable, and this is what you would use to proportion your front-to-rear braking force from a single lever.
With that single lever and front caliper I can stop that heavy bike from 30 mph with one finger. The pressure limiter can be adjusted to skid the front wheel - or not.
I chose the brakes with the plan to add a rear caliper if I felt it was necessary. The lever travels about 1/2" at the tip of the lever from rest to full braking. Adding a second caliper might increase the total travel 25%, but would make the braked that much easier to modulate.
Adding a second caliper to the system would require a longer banjo bolt for the master cylinder, something that should not be too hard to find.
By using a pair of calipers with pressure limiters you should be able to balance the braking system. The scooter parts are more robust than bicycle parts, and the single front caliper will stop a 700 pound scooter from 40 mph tothe limits of the tire.
Hope that helps