Not heard of anyone growing leeks in root trainers and potting on.
I have always grown leeks in deep containers.
Last few years in deep pots, that are 9 inches deep.
Sowed mine Feb 14th in greenhouse.
Up nicely now and would expect to plant out late May and early June, when pencil width.
Here are mine growing today in the greenhouse...
I always give one of those pots to my Dad too, when ready for planting out.