The only season long bloomer there is the agastache, although the black eyed susans (Rudebeckia) are half the summer through fall...I'd be worried about lavender sitting cold and wet mornings
Some salvias can be made to bloom for long periods through continual dead-heading; I love S. 'Caradonna' for its dark stems that make even passed flowers look good from a distance. 'East Friesland' and 'May Night' are also proven in my garden and I do like them but don't consider them all season plants...(and knowing how much Wolfgang Oehme hates it when we Americanize names, especially German ones, I will add that the correct cultivar names for those last two are 'Ostfriesland' and 'Mainacht'. Sorry Uncle Wolfie...
I love my little blue-eyed grass (Sisyrinchium angustifolium
)which flowers in May-June with sporadic flowering until frost, but that is a small front of the border type and hardly a big show...with a little more shade I would have suggested Geranium 'Rozanne'; a real work horse. And some of the new violas are season long; Viola etain, V. x 'Purple Showers', 'Blue Moon' or 'Columbine' are all first-to-frost types... daylilies have 'Stella D'Oro', 'Happy Returns' 'Pardon Me", all dwarfer reblooming types...
perhaps with an 'Endless Summer hydrangea for bulk?