My personal view is that ultimately, what would be best is to encourage flea predators. So rather than spraying down poison, I would probably go with the strain of nematodes that attack fleas in their egg and larval stage in the soil. Nematodes need to be watered in during season with proper temperature range for the strain.
It might help to create a smaller, virtually flea-free area for your dogs to play in for the time being -- for this I might use diatomaceous earth even though it will kill indiscriminately. Perhaps just the area immediately outside of the door. Temporarily fence specific areas.
Your lavender, lemongrass, and marigold will grow in just a pocket of better soil you make for them, but you will need to individuality protect them from your dogs (and anything else) with a circle of fencing or something. I think Rosemary and Sage might also be good additions, maybe eucalyptus, too?
Mint as mentioned prefer more moist and somewhat shadier location, and I think the thing to do is to place them where water will collect -- lower at the bottom of any grade or slope, or where the rainwater from the roof gutter downspout drains and spreads. I can't remember if pennyroyal which is related to mint is toxic to dogs, but I remember it as being a flea repellant. Have you considered lemonbalm, too?
Don't forget a soapy tub by the door in which to wash your footwear and maybe your doggy paws, too.
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