In my experience 0 percent of people who downloaded my free models bought one of my models at a later date. It just doesn't make sense. You are doing the right thing by creating only 3D prints because regular models don't sell anymore for the same reason.
There are too many free regular models and people can get everything for free. If you want to give away free models only give them away to customers or people you know are interested in buying your models or want to show a free sample to their boss because they are buying on behalf of the company. All you need to do is state that you will offer free models upon request on all your model descriptions then the people who really care will come to you and ask.