If there's any play between the bike and the trailer, they'll kinda do this spring thing boioingboioing because of the pedalling not being 100% consistent power, now if this would also trigger breaking it'd feel very terrible

Those "surge brakes" suck. They suck on a car, and they will be even worse on a bike. There is a reason most trailers have electric brakes and require the towing car to tell it how much brake force to use - it works much better. (air brakes might be even more common for trailers, but that only applies to large semis - I don't know enough about air brakes to comment but they need to be acknowledged) The only advantage of surge brakes is you don't have to ask if the towing vehicle had the right controls to support them, which is why rental trailers mostly have them. However for every other purpose electric brakes are better.