When I get an infestation of aphids on either garden or ornamental plants the huge majority of the time nature takes care of it with predatory insects and the aphid attacked plants recover relatively soon.
Not sure if this is what you may be seeing in your zone 6, but mite activity has started to spike here in MY zone 6. Are you seeing mite activity in the top of the plant? It has been my experience that marigolds are sometimes "mite bait" and I have had enough "imported" mite problems from transplants on tomatoes that I wouldn't even try marigolds with tomatoes. I admit, may be different in different climes. Mites of various species tend to coalesce on the undersides of the top foliage.
I think companions depend on climate and region, but sure work.
hortster

Some Marigolds, because of the scent, can aid in so confusing some insects that they leave the target plant alone but since Aphids tend toward newer, lusher growth, and since that is usually at the top of a plant, you most often find Aphids at the top of a plant. In some 50 odd years I've not heard of someone making a tea from Marigold leaves and spraying it, but a simple sharp spray of water (a hand held sprayer works quite well) will knock the wee buggers off any plant.