Milkweed is the BOMB. I have never made a bowstring out of it, but I have messed around with it a lot for other things. It is very fine and silky, and so, so strong. I made a fishing line with it once as a kid, and even caught fish.
Double reverse twisted, using quality, long fibers is the way to go. It's just the nature of the beast. Plant fibers, even very long ones, just want to be bundled up together and work together. To improve strings, I think the path lies in making multiple smaller threads into a big string. If you are good, they can be sewing thread sized, but I'm not that good.
And, the flemish loops are probably the best method to avoid knots.