I love 10m trot circles on tanky horses, because you can just pretty much drop contact and they cant run away because they are stuck in a tiny circle. Its a nice way to be able do a big soften without losing control.

You dont *have* to do the canter circles 15m, you can make them slightly larger, maybe lose some marks, but whatever. Better to do a nice 17m circle than a crappy 15m one.