Not sure how well that'd work in your situation, but maybe you could license the pro parts with one of the licensezero ( https://licensezero.com ) licenses. That would disallow use of your software (or the parts of it you choose) with either closed source software, or in a commercial setting (depending on the license you pick).

It would mean more work spelling all that out to your users, but you would make up for it by not having to hide/separate code as much. You'd have to be comfortable with 'showing all your code though.