There’s a related product (can’t find the link) that has a physical connection to the bike that includes a load cell that drives a motor in the trailer to keep

There’s a related product (can’t find the link) that has a physical connection to the bike that includes a load cell that drives a motor in the trailer to keep the net force (when accelerating or braking) at zero. In fact if you know the gross mass of the trailer you can get pretty close with just an accelerometer, but I don’t know of a commercial product that does this.