Why are onscreen keyboards better for you? Is it because they are smaller and less movement? Is pushing a button harder than touching a screen? Have you looked into thinks like corded keyboards?
Python is probably the easiest to use with audio typing because of the lack of non-alpha-numeric keys. I think you are on the right track.
I know a dyslexic Physcology professor at an engineering school that dabbles in coding and is a big Dragon user. I'll ask him.