Don't give away too much or you'll find that people won't go with the paid version and you'll still be stuck supporting thousands of people with no equal compensation for your time in protecting their sites.

Been there, done that. Also been there and done that with the donation model. About 8 years ago I deleted a plugin from the WP repo because I received maybe $5 in donations and people were demanding support and getting angry when they didn't get it as fast as a McDonald's drive-thru window.

It's a long-standing problem with WP, people get a load of stuff free and then complain if they have to buy something - even when the price is less than a cup of coffee per month! And they complain when they can't get immediate support for free software.

These days I simply sell plugins, nothing free. Either they want it or they don't. And they get support too because that's what they're paying for really. That model works!