hau Patrick, If you fill out your profile on the control panel, we might better know where you are on the earth mother and that means we can taylor answers that fit your situation.
Since I have none of that type of information (look at my profile under my name to know what we need) I will give you some information about your ideas.
By the way, if you investigate a large area around your location and see no tree lines that might be used for living fence, that can be an indicator that it doesn't work.
Especially if you are in Europe where such structures are still around from the middle ages.
Fast growing trees have the very real tendency to be short lived trees, or at least far shorter lived than the slower growing hardwood trees.
If you can get them two of the best are Honey locust and Black locust, both are very rot resistant, grow at a nice pace when young and then slow down as they get older and they both coppice well too.
The wood is one of the standards for wooden fence posts in the USA and other countries where they are growing (only the USA is the natural habitat for these two trees.
One other tree, the bald cypress is known for it's non rotting abilities and in swampy areas it is the prime choice for fence post wood.
Now, about the need to go deep, what is it that you are keeping in/out of the area you want to fence? That has a great deal to do with how deep a post needs to be sunk into the ground.
Surviving without water has a lot to do with your soil and the microbiome in that soil, most tress will need to have some water in the soil most of the time.
If a tree is living, it will be taking in water, no matter what you do to it; prune, coppice, pollard, espalier, etc. if it is living it will need water, so cutting a tree back to give other trees water availability, is more one of those "Only on the internet" types of thing.
Depending on your latitude, you might be able to use moringa, a nicely growing tree that is one of the best sources of protein for humans that is available.
The moringa might work fairly well to your needs and it can also be coppiced.