Your salads sound healthy and productive; I would keep the cloche on untiil April, then give the whole area a good watering. Mizuna and mustards will probably be rising to flower by then, while the lettuce and wild rocket will keep producing leaves until June. You could leave one lettuce to keep seed from in September, or remove all remaining plants to make way for June plantings of beetroot or french bean of fennel, leeks etc.
Lots of great possibilities.