Can you get dual piston cable brakes? The main issue with my cable operated disc brakes is that they only operate on the outer brake pad so it basically pushes the disc against the inside brake pad.
Although it works, it's not an ideal system as the disc gets warped every time you apply the brake unless you stop them up very carefully.
I've seen dual piston cable on Ali.
Why don't you try
these? They're a direct swap dual piston hydraulic caliper like the TRP ones Woosh suggested in post #2, but are only £20-24 a pair.
They're not full-monty Hydraulics, but better than the usual cheapy single piston cable caliper, and use your existing levers/cables so retain the motor cut-out. I'd think they'd be perfect for your MiRider, as they are for my D11 folder.