Now I do agree with you...yet (playing devil's advocate for both or us) if we make defense spending a voluntary thing, then in times of peace the funding would likely be low, and then when you need the military (say you were attacked) you likely would be caught without sufficient funding...and no time to build up (even if you could quickly raise the funds).

Not defending the military waste (at all) yet I see in this example something that would need proactive funding. Maybe there are other (better?) examples, but I do think there should be some limited central services...

I do struggle with this though, as I think (almost exclusively) politicians are corrupt, and the tax system is being abused...but I'm not sure we can do away with it all completely.... 

 I think the amount of people who like to collect guns & play war games, with basically all govt restrictions on weapons ownership removed, would probably serve as one of the greatest deterrants ever. Red necks are currently viewed as a serious threat to our govt despite being seriously crippled by regulations on what they are allowed to own & our govt basically being the most heavily armed govt in the history of the world.

I think drone tech & open source Ai tools & cryptography are really only going to make the economics better for individuals & defense vs larger institutions & offensive measures in the long run. 

 Exactly this. You would not try to overthrow a country where its citizenry is heavily armed, because they will defend themselves. Meanwhile, the US military does not even exist for defense, it is a purely offensive force. And a disarmed citizenry with a centralized power center is a relatively easy target - you just have to take out or subvert the leadership.