Most St Aug is coarse bladed and will plow right through other grasses including bermuda. All you have to do is mow it at the mower's highest setting. The only thing that gives St Aug headaches is broadleaf weeds like clover, oxalis, horse herb, and even the lowly dichondra. There is a finer bladed variety, but I have no experience with it. I would think the finer bladed one would have more trouble crushing the other grasses. Practically you will be limited to what you can find at your local nurseries. If you get one piece of it and put it down in the shady area, it will spread about 15 feet per year to cover the rest of the yard. That is how I am spreading it in my yard. It takes a lot of patience, but you already have a nice enough turf. I'm dealing with prairie grasses and bermuda (ugly). With your more moderate temps, it might spread 20 feet per year. Mine stops spreading when the temps get into the high 90s every day.

SoCal is not a good location for fescue. I realize they sell literally tons of Marathon 1, 2, and 3, but I maintain it is contributing to the water shortages across the state. If you lived in La Jolla, then fescue could work, but when you get east of I-5 then it's too warm and takes too much water.

Watering: Deep and infrequent is the mantra for watering. This is for all turf grass all over the place. Deep means 1 inch all at one time. Put some cat food or tuna cans around the yard, and time how long it takes your sprinkler(s) to fill all the cans. Memorize that time. That will be the time you water from now on. My hose, sprinkler and water pressure takes 8 full hours to fill the cans. Your time will likely be less. I like gentle watering. As for watering frequency, that depends on the daytime air temperature. With temps in the 90s, deep water once per week. With temps in the 80s, deep water once every 2 weeks. With temps in the 70s, deep water once every 3 weeks. With temps below 70, deep water once a month. Note that you have to keep up with quickly changing temps in the spring and fall. This deep and infrequent schedule works in Phoenix and in Vermont, so it should work for you. The reason for deep and infrequent is to grow deeper, more drought resistant roots and to allow the soil to dry completely at the surface for several days before watering again. If it rains, reset your calendar to account for the rainfall.

I'll buy like 10-15 pieces and put them all around the yard and watch it spread. It's cheap and putting down that many pieces shouldn't be too much work.