I was just thinking about this the other day haha.

I like your idea of the trailer being automatic in a way. I also like the other ideas mentioned here and they could be combine maybe.

Like some simple bump switch logic where it first checks if the trailer is relatively straight. Them if the trailer is being pulled and the slack is taken up and a bump switch h is pressed it starts accelerating. Then if the trailer starts pushing on the bike it will slow down.