Tall fescue it what's typically recommended for drought resistance. I can tell from personal experience that it's drought resistant but not drought proof. If you have a couple of weeks with no rain with temps near 100, it will start to die off. Zoysia is another possibility. I'm not sure if you can grow it where you live, but it can take summer heat and drought.

Review your many watering practices. Pick the one that looks most like this: Water deeply and infrequently. Deeply means a full inch (as measured by a tuna or cat food can). Infrequently means once a month this time of year gradually changing to once a week during the hottest part of the summer. You'll have to measure your own sprinkler to time how long it takes to get an inch. Mine takes 8 full hours. My neighbor's takes 20 minutes. Once you know that information, try watering a full inch and adjust your water frequency according to how long it takes your lawn to look withered. Don't water just because you haven't watered recently. Water when the grass looks dryish. With St Aug that is a fragile time. Water at the first sign that any of the lawn is getting dry. Mow St Augustine at the mower's highest setting. If you can go 2 or 3 weeks in between mowing, do that. I have a St Aug yard that has not been mowed in 14 months and, if you like tall grass, it looks great. It seems to have stopped getting longer at about 30 inches tall. You don't need to do that but I can tell you that 2 inches is way too low, 3 inches is still too low, and 4 inches is lower than I like. The taller the better especially out in the full sun. Do not ever till your lawn. If you ever get that urge, stick your finger in a light socket until the urge goes away. If you till it you will be mowing a progressively bumpier and bumpier lawn for years to come. Jackalleness is new to this forum. He lives in Oz and apparently is not familiar with St Augustine. It does not grow from seed. Maybe they do things different down under.

It sounds like your options are pretty limited. Fast growing trees are going to get really big. Most fast growing trees also end up being weak-wooded and short lived. 'Green Giant' thujas grow quickly and have far few problems than Leyland cypresses (which I would definitely NOT recommend), but the Green Giants are going to get pretty large eventually (and very possibly much larger than you'd want). You may want to consider a vine-covered fence or even a hardy type of bamboo with a root barrier.

I think it was me that mentioned vinca and your point is well taken. Vinca is a major problem in many woods. In my own 1/2 acre woods, vinca minor, pachysandra terminalis, and galium odoratum were planted by a previous owner 40-50 years near the woodland edge (but in the woods) and they haven't been a problem in that they form about 100 square foot patches each, separated by paths, and there is no sign of any of them across paths. Perhaps it is the colder climate (US zone 5), rocky soil or greedy maples. Probably the latter as the woods in this area are almost all maple (sugar and norway, which have interbred). I welcome those patches as reprieve from the garlic mustard. In fact, for some reason in my woods, the vinca coexists with trout lilies, jack-in-pulpits and other natives, while I've had almost no garlic mustard in that area. On the other hand, as I remove garlic mustard from other areas, I've planted/propagated native species (trillium, bloodroot, mayapple, mertensia, and many others) although, unfortunately, none of these are really ground-covering in my woods so battling GM is ongoing. I am also slowly trying to reduce the norway canopy, planting hemlocks and red oaks, and if this is ultimately successful, perhaps that patch of vinca which seems fine now will become a problem.