And in the system I advocate, you could say that, while in the system you defend, you cannot. Or you can, but it wont fly, of course. Find a way to refuse (difficult in europe, where the money is nearly always taxed before you get it) and you will eventually see why we say the power of the state grows out of the barrel of the gun.
If it's completely optional, in the (for the lack of a better expression) "heat of the opportunity" 500 of the 1000 will refuse because they really need the money for something specific right now, and they already helped some guy last month, and, and ....
Another 300 will refuse out of greed, another 150 will point out that they have a choice, and this doesn't match their ideology or they don't feel compelled enough morally.
So my share grows twenty-fold, can't give that much because I also divert a share of my income to maintaining our roads, electricity infrastructure, military, etc. And for some it snowballs into reducing their willingness to pledge help because it leaves them with too little disposable income. You go hungry.
Solidarity requires broad coverage to work. Voluntary charities don't cut it, at least there is no example of sufficient equivalency out there, e.g. funding a national education infrastructure or healthcare program. Also, the overhead of your proposal leads to higher bureaucracy if it is to handle a similar volume of money assignment.
But why should I try to refuse paying taxes? Excuse the crass example: try to enslave people, which might be good for your personal profit if you could keep it that way (and "some" people would do so if it were optional)... you'll still get stopped at gunpoint, presumably by the government.
I think both enforcing taxes and personal freedom is justified ethically and in terms of the net benefits, and accordingly required/guaranteed by law.
One paradox you face is that while a relatively wealthy nation with a culture that values work and production can maintain a welfare state and pay for it with taxes without destroying your productivity overnight, you still sap the very qualities that make this possible in the longer term. Which is to say, over time doing this makes you a less wealthy nation with a culture that places less value on productive work.
I require proof of that. Clearly you can't give it for now, but I'd like to point out that this is a problem of blame attribution: you see the cracks (which are always there in some form or another) and attribute them to a certain ideology you're compelled to oppose. I'm guilty of this too, it's human nature, but still not a good argument when reasoning about the merits of a system.
As for your proposed alternative, I see how Libertarian state could work in theory, but not under the requirement that it matches a metric like the GNH (yes, I'm actually serious about that) of current states. As mentioned, IMO absolutely not worth aiming for.
With regards to the "USA protecting the world", I wouldn't say the out-of-bounds spending on your military-industrial complex is benefitial to Europe's safety. Apart from that, Europe is easily second, so there's only one entity in the world we'd have to watch out for ... ;)
On Sweden: they have utterly failed in their immigration/integration policy. You can blame it on the conservatives if you like, but claiming to see the writing on the wall for their whole system is far fetched. See blame attribution above. And you'd have to elaborate on the Swedish educational system being libertarian in nature, because most Americans so far have called its current form socialist (or communist, you get my point).