Okay, I just dipped into your timeline - 2 cents.
Every 4th tweet should be your art. Repeat from your entire portfolio and show side by side sketch/finish if you can. Show published work. Shout out clients.
Do. Not. Be. Shy. Your followers don't see everything -they see a fraction of your posts. If they love you they won't care that you repeated a thing 3x a month. They won't even if they see every repeat and they won't.
Every 20th tweet (once a week) post that "commish open" and price pic. Rotate them. See which one gets the most clicks.