Yeah, exactly what I was fishing for. Go fast not the best idea for push trailers, and then if to go slow, you would want to pick a slower, or rather just more normal 408 wind vs a fast motor.
With less load, a 404 would be a good hill climber in 16" wheel, able to take about 1500-2000w for a decent enough time period. 72v 25 amps kind of thing. But total weight should stay more like 300 pounds max for any of the 500w rated motors.
Could still work ok though, if the push trailer is the second motor. Then its half the load, and two motors will let you cruise up the hill at 15-20 mph, and the push is just helping the bike motor. So you have the pusher just taking you along at 15 mph or so on the flats, but a geared motor on the bike coasts at that time. When the load gets big, you turn on that second motor. But as Chalo said, you might find it unstable at even lower speed, fully loaded.
But for sure, wrong motor IMO, for the kind of 400 pound load typical of a rider, big battery, and a big load on the trailer.
Complicated,, What really works best is usually just a big enough motor to go fast up that hill easy, without waste heat, even when heavily loaded. I did my touring on a 5304 crystalyte rear motor on 48v 40 amps. 2000w up the hills kept me going 15 mph up the hills. To extend range, 15 mph on the flat too, just ticking along on 300w.
That bike would do 32 mph though, when I had the range to go fast. And it pulled my bob trailer full of groceries lovely, at 20 mph or so.