braking considerations aren't for slowing down under controlled conditions - they're when you have to slam on the brakes due to some unforeseen circumstance - it's at that point the weight of the trailer - which up to that point had been manageable - becomes a serious issue, as that heavy unbraked object either tries to run you over or pull you to the side.
Clink & Clack the car guys tell the story of a couple of "mooks" who were transporting some large sheets of plywood on the roof of the car - one driving with one hand and holding the sheets with the other, and the passenger holding the sheets on the other side. They accelerated slowly, cornered gently - everything was going great, until someone pulled out in front of them, and Driver Mook had to slam on the brakes. The previously-manageable plywood shot forward off the roof, taking the hood ornament (and some of the hood) with it.
Everything is under control until it's not.
You want the trailer to be doing at least some of the braking. Since you're clearly able to design and fabricate, I would look into some form of rider-operated cable brake (cable would allow you to incorporate a cable splitter for detaching the trailer). If the bike is going to be dedicated to this purpose, you could operate the trailer brake off the bikes rear brake lever