It could be specified that only the free version is open source. You may also experiment with open sourcing some paid features too, possibly with some delay aft

It could be specified that only the free version is open source. You may also experiment with open sourcing some paid features too, possibly with some delay after the release if you're concerned about forks eating into your profit. I think the fear of forks is often overblown in this context, especially with consumer facing software. You have a huge advantage in terms of expertise and an established user base, and people will tend to pick your project over a fork, even if you open source everything and publish a paid package from the open source code. Those who are usually willing to pay for software will buy it for the convenience of not having to fiddle with the source code after every update, and to support the development of the project.