On past experience I don't think that strong root will get out of hand. The others are growing strong and should keep up or exceed it if given the chance and you still have many years of grow and develop to take this tree from pole to bonsai. Still plenty of opportunity to work on the roots so I would not pull it out now.
By the look of all those young roots this tree had a hard root prune last year or the spring before. It already has a pretty good radial root system but I like to see even more ramification so I cut all those new roots around 1 or 2 cm from the older growth something like this:
I know the board thing is cool at the moment but in my experience, after a good root prune like you've done, there's no need for a board. Even planted straight in the ground you'd get very few down roots when the tree already has so many strong laterals. Any down roots that do develop can be removed at next transplant. Eventually the tree doesn't bother growing new down roots.
My potting mix contains fertiliser so I'm essentially fertilising from day 1. You'd need to be using something real strong to damage roots so I'm confident the wait x weeks is another myth. It works because there's always some nutrients in a potting soil and most trees can manage for a few weeks on stores anyway.