I have grown this variety for years in my zone 3 garden. I started them from seed as you did. The problem with direct sowing is the length of time they need to become a viable plant. I'd say save your seed and start them next year.

This is a very short lived perennial. It blooms so profusely that it actually blooms itself to death. I cut off any blooms about 3 weeks before frost to strengthen the plant. Even with that I still save seeds and plant them every couple of years so I always have this lovely plant

Here in zone 6 I direct sow coreopsis yearly. I literally prepare the soil and broadcast the seed then water a bit, that's it, germinates in a week or so. Now this was with some coreopsis variety that was grown by my grandma and she gave me the seed. Note that coreopsis is incredibly easy to start indoors as well. You're not late, start a few inside under heat if possible and they will germinate quickly.