I think I would take the trike to a empty parking lot and play with it. Do some emergency stops and get used to how it feels and just leave them alone.
I have played with changing brakes on several bikes before, generally I always find that one thing leads to another and on one I ended up with too much brake force and the rear would lock up very easily.
I think linked brakes are a good thing. Unless you are a road racer and are very young and skilled with good reflexes and daily practice, the linked brakes do for you what you should do.
You do not want to braking force to be enough to easily lock the rear up in any event, should be to where you have to really work at it to get it to lock up.
I even had another bike that had a very touchy rear brake, tried this and that from a worthless TBC device to an automotive proportioning valve I installed inline to the rear brake to cut down the amount of fluid transfer to the rear caliper. That worked somewhat, but you could still lock the rear wheel, it just slowed it down a bit so you had more time to react.
I would just go practice a bit, figure out the limitations that you have, if any, and be aware of them. Might be it stops just fine with a little bit of a heavy foot.