Unlike the old days where you could get progressive braking applied by tapping into the brake lines (my old 1982 C30 had that), these days most decent brake controllers have inertial sensors in them and can provide all the proportional braking control you might need.
Basically, there are 4 wires you need to provide to the brake controller. Ground, +12V (high current, I want to say 30 to 45 amp range, but follow the directions for the controller), brake signal (a relay triggered by the same switch as the brake lights), and then the output to the brakes (again, high current rated).
So the advice: get any proportional brake controller you want that doesn't need to tie into any brake lines (hydraulic or air), and install according to the directions included, and you should be good. This is the one I use in my SUV: https://smile.amazon.com/gp/product/B017NAH5WK/
As I've jokingly told many people, I'm not worried about my 7000 pound trailer affecting my 20000 pound bus at all, but if it does, then having functional electric brakes wouldn't have made a difference. However, at least here, any dual axle trailer or trailer over 3000 pound GWR is required to have it, and the tow vehicle the controller. So, for the sake of legality, I will add it, then dial it down to "barely does anything" because I don't want to burn up the trailer brakes trying to slow down a 20000 pound bus.
And 20000 is what it weighs empty, pre-conversion. I'll stay under the GVWR of 35050 easily.
When towing that same trailer with my SUV, I won't leave the yard without verifying the brake controller is working, and as soon as I get onto asphalt, I do my setting adjustments. With the bus though? It is difficult to even realize the thing is back there.
I hope this helps,