Ask-a-Dave's-Gardener: Puzzling question for choosing the correct plant, 1 by LanfrancoLeo

LanfrancoLeo wrote:
Hello guys!
I need some expert suggestion about a puzzling situation in my garden.
Few years ago I built a nice walkway in my small backyard garden with some concrete stones. The walkway is a narrow straight line 21 feet long x 3 feet wide (approximately). Every year I use to plant boarding annuals along the walkway. I use decorative flowering plant of an height not superior of 3 feet that bloom all the summer long. This year I decide to make a great improvement on my walkway by planting some boarding perennial that must follow some specific requirements:

a. Th plant MUST be a good bee plant (my garden is a pollinator oriented garden, ALL the plant that I have must feed bumblebee, otherwise I will not use it!!)
b. The plant has to be relatively drought tolerant , I live in central PA where the rain is abundant, and for an ecologic reason I want some plant that can be happy with a single or double good water irrigation per week.
c. The plant has to be no taller than 3 feet
d. Has to bloom continuously from late spring to early fall.
e. must be suited for a full sun position
f. Cold hardy in my area ( area 6-7)

In the past i made some trials with mixed feeling result with the following annuals:
a. Impatients: very good display, really majestic effect, but require too much water in full sun and by the end of september the plant die inexorably, moreover the plant was not bee friendly.
b. Marigold, I used a single flowering type, the plant was very resistant to the moderate irrigation regimen, bee friendly, but marigold have the bad habit to drastically reduce their bloom during the hot summer month, where I need the most for feed bumblebee and for aesthetic purposes.
I was thinking on two possible options:

A)COREOPSIS VERTICILLATA (Tickseed, Big Bang Series )
B) LEUCANTHEMUM (Victorian secret)
However the problem with coreopsis is that the flower are relatively small, although colorful, and the plant are not usually that big.
And Leucanthemum has a very good flower size dispay BUT need a good deal of deadhanding (that become very difficult with a stretch of 2 X 20 feet garden border) and does not provide a good alternation of color (it come just in white) that I would prefer.
I know there are quite a bit of requirements, but for this reason I am asking to you guys considering your superior knowledge and bigger experience in gardening.
I attached the picture of my walkway to gave you an idea what I am talking about...