Hi Ken,
Moving up to larger pots will help. Your potting mix is too porous to hold moisture. You can purchase the new moisture retention pellets and mix them into the soil. Or if you have some clay soil in your yard mixing that into the mix will help some also. Clay soil retains moisture longer that all other soils. There are also potting mixes with the moisture pellets all ready in them but I think the quanity in the mix may not be enough for your conditions.Good thing you picked marigolds, marigolds are a good choice, stick with them until you get this figured out. Don’t try to grow anything other than plants that like heat and dry conditions. Impatiens would have probably died on you.
Your plastic pots are a good choice too, clay pots would spell disaster. They dry out too quickly.
Merry Christmas Ken,
Diane