When I finished my DM internship, the advice I was given by my instructors was that if I wanted to be an instructor, work as a full time DM for a season or two. Where I was at, thats 3-400 dives a season. I think that is by far the better way to go for several reasons. First, thats a lot of experience you are gaining. You are also getting paid, meager DM pay, but still better than paying yourself for instructor internship. Lastly, after one or two seasons working the dive industry, you'll know if its what you really want to do or if turning a hobby into a career would be a big mistake (it almost always is).