Gomphrena questions

So I've always read that gomphrenas love it here and bloom all summer with little to no supplemental water. These east-facing gomphrenas were doing great until about the beginning ofAugust but then their leaves turned yellow and now they're about dead.

I've been watering them about once a twice a week but that hasn't seemed to help. Is there anything I could have done differently?

Also, if I want seeds can I just chop them up and leave them in the raised bed or should I try to harvest the seeds from the flowers and save them indoors over the winter?

Shoutout to the lone zinnia carrying this raised bed.