I guess I'd go with okra in your zone. Why not have a crop you can eat and enjoy?
I don't like amaranth because if you let it go to seed, you're in trouble.

Actually, I've also had okra and hyacinth bean self seed. But, all, including amaranth do great in the heat. Just have to chop down before it reseeds.
I haven't eaten amaranth yet because we have so much chard. Will have to try the greens this summer.
We're moving to NC in July. Will have to try amaranth grain next year.
I have 4 rows of okra. I love it but not hubby. I'll probably pickle a bunch and give a bunch to my family before we move.

When I gardened in 7b my favorite summer cover was purple hull crowder peas mixed with orange sulphur cosmos. They looked great together, and the beneficial insects loved them.

I agree with purple hull crowders - or any field pea. You might try sunn hemp, which is also nitrogen fixing. Okra encourages nematodes.

Spinach is very fresh for vegetable. I am very happy to consume spinach. How to plant is too easy. In the summer, we need to make a cover on it like using the plastic on top and spinach are often splashed with water.

Here is a link that might be useful: vegetable gardening

When I was in a hotter climate I used Cow Peas as a summer cover crop. They shade the soil, smother weeds and the pods are excellent. Seed is also much cheaper than Okra.

-Mark

Hairy indigo and velvet bean are good hot weather smothering crops for 9b, both do well for me in fla.