I think you are missing a big opportunity with SexSim.
Instead of selling addons for a small fee, you should setup an in-game marketplace where users can post there own content and charge for it, with you taking a 30% cut. With 18,000 users+, I'm sure you'll see quite a few users willing to contribute content. That would mean a much bigger gaming experience that will attract even more users, and it frees up your time to improve the game engine.