If you are freee of rabbits (which adore small leek plants more than large ones) I would plant them out, indeed I planted a few such leeks yesterday. It is good weather for them to establish, before, perhaps, it turns dry again.
The question for me is more to do with leek moth and I just don’t know if there may be an early flight in May; even if there were, the plants may resist more strongly if they can become established in soil now. I am not covering mine as all my mesh is on salads, but you could cover them with mesh.