I sow my main leeks indoors in deep trays (large cat litter trays, thanks to advice from DD some years ago) quite thinly in four rows. They germinate at about 15C but after that they go in a frost free greenhouse until they can be hardened off and go in a cold frame (usually in March) I use a mix of reconstituted coir, multipurpose or sieved hm compost and vermiculite, which is light but has little nutrients however
They get fed with whatever liquid food I have to hand, diluted to half strength, once every two weeks, upping this to once a week when they get to about six leaves
I plant them out to follow first early potatoes, but they could be planted out sooner if you have space.
They do get to around pencil thickness and are relatively easy to separate from each other if I tip the whole trayful out at once. As I trim the roots before putting in the hole, a bit of damage does not matter. I pinch back the leaves to stick up by about two or three inches for the hole once they are watered in. That way I don't lose any by washing them into the hole below ground level
I feed the soil with Blood Fish & Bone prior to planting, and again early September
I use enviromesh over them from planting through to November to save damage from leek moth or si ilar beasties
And that's it.