Is there a way to make stuff drawn with _draw() (circles, lines, etc...) anti-aliased?
AFAIK, there's no way with godot 2.1.x
with a shader, maybe?
Not even with a shader.
What I did once with particles, is to put them on another viewport and apply a shader on the sprite that used it as texture, that may work with custom drawing too.
If you are going to use a render-to-texture, you could render whatever you want in a texture twice as big, and then draw it at half the size, which will replicate anti-aliasing by drawing subpixels (in a naive way though^^)