To the best of my knowledge, there is no button available to toggle on/off all UI. I think that this would be nice.
Perhaps, even an option to keep different UI windows "invisible" while they are not in use. When I am in combat, I want to see my combat log, but when not in combat I want that area of the screen to not be filled with a giant black box (empty combat log window).
Same goes for squad/target/etc windows. Perhaps this option would be best as a checkbox for each UI window (there are some windows that I always want on my screen even if they aren't actively in-use - bot status, modules, radar).
Thanks for making a great game.