Those self weeding papers are very expensive. You might want to consider ordering plastisol transfers. If you want to use heat transfers that you create, Ironall/Everlast seems to be popular, but to avoid extensive cutting, you may need to make your designs rectangular and just trim using a rotary cutter and cutting mat with a straight edge. I'm using pigment inks with Ironall with good results.
Some advantages of the Ironall transfers on dark shirts versus
JPSS on lights is that there is less pressing time, ink doesn't contaminate your parchment paper on the second press, and dark shirts don't show mistakes if your teflon paper gets dirty. White shirts show everything and find more waste doing those versus dark shirts.
I just did 500 pieces in a week on light and darks and have a whole list of advantages of each style and tips to save on time and prevent waste of materials.
PM me if you want more info.
Come over to the "dark" side, it's not that scary!