If you can afford multiple speech therapies then you must give them a shot cuz they are the last hopes. At least you gotta learn how to use 'smooth speech techniques' to reduce tensions onto your articulators to avoid stiffnesses. I think stutter can be boiled down to abnormal tensions forced onto PWS' articulators so that we stutter out of control due to our neurologies. And gently articulating can somehow reduce tensions to mitigate stuttering to some degree. Sign language is just an expedient. There are very few normal ppl that can understand sign languages and if you give all hopes for sign language, you will trap yourself into another abnormal social circle where only mute ppl are likely to understand sign languages and you will get frustrated like hell since you choose to communicate with mute ppl cuz you are just speaking disfluently while they cannot speak.