Ben,
instead of placing plants into the normal nursery pot, we shift from say 4" pots to a 5" diameter saucer that is about 2" deep. Keeps the roots growing flat, and easier to shift into the eventual bonsai pot. Plus the plants seem to prefer the saucers over the normal nursey type pots. The saucer's shape might mimic how they grow in the open ground.
By the way there has been no problem with very shallow pots and the supposed retaining of extra water. [ 1'' to 1/4 " deep pots.]
Just finished shifting 5 Sageretia from saucers into bonsai pots, the root density was impressive.
Bonsainut, we count on the trees being potbound before the heavy rains, simply because there is no zone of unused soil for water to sit in. However the mix being used also remains freely draining right up to the time of repotting.
Good Day.
Anthony