Try keeping it a little bit looser. Embroidery thread already whips like the devil, but it’s tension is tricky. I find that if your just a little bit looser than what you would shoot for with Polyester you will come out just fine.
I twist my string a bit less than hamstring tight, but a bit more than bulk poly. Should I be aiming to get a twist around bulk poly or even less? I will be experimenting tonight
There are many good options available. Personally I love how smoothly the mara 100 thread by Gutermann breaks in. Keep in mind that on string usually takes about 16 passes (16 threads) which can add up to about 50 yards a string. Wooly Nylons tend to be incredibly smooth, also silk can be very smooth, and provides one of the best controlling weights for slacks. If you want to cross over to embroidery thread Gunnold Embroidery thread is in my opinion the softest embroidery thread.
Probably this poly sewing thread I get from a website. I use it to make my type 1 string. Its really soft, yet whips well. Also maybe Coats&Clark’s Dual Duty
Not to sound like a smarty pants, but coats&clark covers just about every kind of thread there is. :Upolstry Nylon, Dual Duty XP, Embroidery thread, Quilting Cotton. All kinds of stuff.