I can recommend Fast Flight plus. Haven't tried the other FF spin-offs, but there is definitely a big jump from B50 to FF plus. From making the string to shooting, it is a completely different animal.
I have to caution against linen string from pyro supplies. I got a spool that tested at half the breaking strength advertised. It was well recommended, so maybe I just got a bad spool. We now use it as trellace material for our scarlet runner beans.