Your timing is perfect. There are always a few fish at the mouth there. The run occassionally arrives early, so with some luck you could get into a bunch. Benefit to going then is the hoardes of anglers arrive in August. I live very close to the Deschutes. At the mouth there are runs already described earlier, but of course there are sweet spots within those runs. You will learn those quickly because anglers stay in them for long periods.
Honestly the best spot that time of year is the island next to the campground. Fish the east side towar the top end with spinners. The west side bottom end can be good too. They do not limit boat permits for that little run over to the island. The reserved permits are for drifting the river on multi day floats.
Water temp can be an obstacle in July. It can get over 70 F there at the mouth which causee some degree of lockjaw. Most of us stop fishing by lunchtime then to reduce stress related fish mortality.
Full disclosure, I am much better at catching wild redside trout on othe D than Steel. I would be glad to show you upstream water to catch a few trout with small spinners, or potentially quite a few if you bring a fly rod. Send a private message if that is of interest, and we can coordinate.