You are correct about the string.
Milkweed makes a super strong string. Good bow strings.
It is hard, well more time consuming that hard, to make a good string from it.
Leave it alone until the frost kills it. A day or two later cut them and lay them out to dry.
It will take a while for them to dry. Once they are dry the fibers have to separated from the main stem and the outer bark has to be removed from the fibers. Most time consuming.
ALL of the outer bark has to be removed. After all that is completed making the string is quite easy.
Cut lots of stems. You will most likely need them for practice.
David