Geraniums? I mean the cranesbill type, not pelargonium geraniums. They pause around end of June or July, and will need to be cut back then, when new growth takes over and more flowers appear. Geranium 'Wargrave's Pink' is particularly long flowering.

Otherwise Coreopsis grandiflora - it is perennial, but it does have a tendency to be very shortlived, needs a sunny, sheltered spot. Erysimum Bowles Mauve goes on most of the year, but gets woody after a couple of years, so cuttings should be taken yearly. It might be a bit taller than you want though.

Gaillardias

I agree with both of the above, Coreopsis are lovely. Gaillardias are one of the longest flowering plants that I have in my garden, the dwarf ones would be Goblin, Arizona Sun or the Arizona red shades. They can be short lived too like the Coreopsis but they are easy to divide and keep the newest parts. They are well worth it!

You could try the saxifrages - they have interesting leaves when not in flower.
Most "alpines" are low growing. Or look at seaside plants which are low to avoid the wind coming off the sea.

If you could stretch to 16", there is a ragged robin called Jenny which has been a complete cloud of pink flowers since March, having had the occasional flower all through the winter too, and having an evergreen groundcover leaf to 4" high.
I would never have bought it but received it as a gift, and it seems to be sterile - hence never stops flowering as it's setting no seed... now I love it!

Also, Thalictrum ichangense makes a froth of pale grey-pink flowers at 8" over interesting deciduous foliage, flowers May - Oct.

Some heucheras flower on and on, although many don't, so check out Plantagogo or Heucheraholics websites.