I repot pines just before or as they become active in the spring. Before the candles start to move. In my neck of the woods that is usually March/April. it will be earlier this year and i am starting repotting this week. This gives the tree the best recovery time as it begins an active growth season. I agree that the first formative years are the best opportunity for creating desireable root ball formation. The colandar concept was intended to air prune and prevent long tap or escaping roots. I think it works very well to a point. I have settled on using the first four years to get as much growth combined with a good start on the root ball formation. I find it hard to imagine too many roots unless they are too long and too thick for Bonsai pots. I try to create good strong short fat basic nebari with as many feeder roots as possible until it starts to lift out of the container and needs a repot.
Basically this is an outline of the timing. Sow seeds January
Seedling radial cut ( 2nd set of needles) approx 60 days ( March ) indoors.
move to 4 inch pot 6 months ( no root work) ( August )
Move to colandar March year one ( selective root work some cutback)
Colandar goes on bench ( air root prune)
March year two ( root prune place back in colandar, put colandar in grow bed)
March year three ( cut escaping roots, rotate and put back in grow bed)
March year four ( remove from colandar, root work to escaping roots, circling roots or strong downward roots. Small amount of work if surface roots need attention or root graft on one side needed, otherwise the least amount of disturbance to the overal rootball ) Usually change surface soil to maintain great drainage. The reason for limited root work at this point is to retain vigorous growth for trunk development. one of the prime focussses for year four to eight.
Note: At this point I abandon the colandar and use either an anderson flat or purpose built grow box. I use the smaller deep propagation style anderson flats for smaller size and better depth. 14 by 14 by 5 inches deep. If i am preparing shohin or smaller pines the grow boxes are 11 by 11 and 4 1/2 inches deep.
These points are specific for developing young pines, suited to this stage of vigorous growth and fast recovery.
My Nursery is facing south and exposed to full sun in a warmer micro-climate.
Hope these notes are of some use.