Hi Phil,
Maybe the old guy had a formula board.
Formula boards are good because they can carry huge fins and sails easily giving you freedom to sail upwind and downwind
For teaching put a small fin and sail on it.
If you get the racing bug on a formula your sailing will improve ten fold as you suddenly get heaps of expert mates to hang with.

Another board to get would be a RSX. I teach my kids with a RSX, very strong.
A starboard go is the best learning board but sounds like your surpass that and the kids learn too quick to get any value out of it. I think the starboard go is sold to sailing schools. Is a great board for that.