This is a massive ask, which is probably why you've had no replies so far. For Spring, pulmonarias and hellebores, for Summer geraniums (e.g. Johnson's Blue and Ann Foulkard), ajugas, Alchemilla Mollis. BUT are you sure you only want perennials and only low-growing? There are so many bulbs, shrubs and taller perennials that would make it into an interesting border rather than something kind of dumpy. With only perennials the bed will look rubbish in the late Autumn and Winter. You also need to consider your soil type.