I only have a few seedling growing but I’m taking a little different approach. If I get enough growth from a first year seedling at this time of the year I’m immediately grafting/label it onto a mature limb. My space is limited so I can see a time that I just won’t have enough room for all the individual seedling.
Commercial breeders, like Markus Kobelt here at Lubera in Switzerland, grow out the seeds for a season, and then bud graft onto Bud 9 or some other rootstock and grow them out in the field. It will bear the third year after the seed was planted, plus he’ll have an idea of the tree’s growth habit and productivity.