To max out all skills you need X( Sorry I don't know the exact number) skill points, and there are total X+22 skill points in the game after you reach highest level, complete every mission, collect all skill points pickups. That means there are more skill points than you need, so feel free to upgrade and don't bother collecting all pickups.
The tiring part is farming the supplies. You'll need very very very many supplies to max out all skills. There are two farming methods I found.
This is during the mission "The Deserter" in Montuyoc. This mission also net you to Cluster Bomber and A Better Mousetrap achievement. If you've completed this mission, you can still replay it.
xWSx Logan made a wonderful video for those who prefer a visualization. Check it below.
So when you near the object, go ahead and kill the mission target marked by yellow arrow. This will cause the game over, and you'll respawn near the road where there is a convoy just 300 meters away from you, and it will start driving toward you. Take out your LMG, shoot the escort and stop the convoy to get 5000 supplies. After that, run toward the mission target and kill him to cause game over. Everything will reset and you still have the supplies you just get. Rinse and repeat.
Fast travel to VILLA VERDE ALPHA in Villa Verde. There will be a convoy on the road nearby. If you take the bike and rush to it you can intercept the convoy just in time. (Don't drive the car, or you'll have to do the chasing.) Take out your LMG, shoot the escort and stop the convoy to get 5000 supplies. After that, run into the forest to escape the alert. Once you escape, fast travel to VILLA VERDE ALPHA and everything will reset.
The supplies on the convoy is randomly appear. If it's not the supplies you want, simply kill the mission target or fast travel again.
The way to stop the convoy efficiently is, shoot the convoy some bullets and kill the co-pilot. Once the co-pilot is dead the convoy will stop immediately. Otherwise you can keep shooting it and have a risk to explode the convoy that will only give you 2500 supplies.