I’ve had many dawn redwoods that have fried in the increasing summer heat when I decided to leave for a few days on vacation. I feel your pain! I think others in this thread are spot on with their advice - stick to your fundamentals (watering) and don’t fuss with the tree. It is stressed, give it time to recover and don’t change too many things at the same time.
One suggestion I might have to help cope with the summer heat is to find a good micro climate in your yard that is favorable to this species. Anything above 85 seems to really torch these trees - you might see if you can find a spot close to other plants or shrubs where the relative humidity is higher and the tree won’t be subject to intense sunlight, winds or other direct summer exposure that causes issues.
When I’ve had stressed redwoods with fried foliage in the past, I found it valuable to get it into a cooler spot (both cooler for the roots and cooler for the foliage) and where other plants were transpiring to help with the humidity.
I think you’ll find that if you treat it right you can expect some additional growth in the mid/late summer into the fall. I find that I get two periods of growth - one in spring and then another push in summer.