Hey, judging by your zone, you should definitely be able to start cabbage and spinach (I just direct seeded some bok choy cabbage today). I would go with something like broccoli raab rapini for the broccoli. Rapini is delicious with edible flowers, leaves, and stems and matures in about 45 days-that way your broccoli will avoid the freeze unlike other broccoli types. Collards take a long time to mature, so I don't know if those would make it past the coming cold temps. You could always try it though....?
p.s. Botanical Interests sells heirloom broccoli raab that is supposed to do very well in zone 7 and 8.

This response is too late to help you this year but incase you wish to try boccoli next year, here is my experiences:

I bought broccoli transplants (Pac Man) from Pike's nursery. I planted them on Sept 1. I havested my first 8 broccoli heads on Oct 28. The next 4 heads were harvested on Nov 3, an additional 4 heads on Nov 10. I have 3 heads remaining. I did use shade cloth for a few weeks while we still had high temps and covered them in Tulle to keep out the pests.

I also experimented with planting broccoli and Kholarbi seeds directly in the garden this year. I planted 4 seeds each week. The first ones were planted on Aug 4, 4 more on Aug 11, 4 more on Aug 18, and the final four on Aug 25. None of the broccoli planted from seed outside has formed heads yet. All of the broccoli is now the same size so planting Aug 1 in the heat did not help getting broccoli any earlier. I guess I should have listened to everybody and started my seeds indoors and transplanted them on Sept 1.

Cabbage - I bought "45 day" cabbage transplants from a big box store and planted them on Sept 1. They are all forming heads but they are small, I guess because of the variety.