I have been working on an Open Source project for a while and its getting popular thanks to ease of use and lack of alternatives.

I am lucky enough to find a number of clients who are willing to pay for a more feature rich version. Does anyone have any suggestions for license templates to be used in such cases?

ie an Open core free version ( free as in freedom and free beer) and a paid ( subscription based ) version

Gitlab's licensing model looks similar and am quite impressed by the way they pulled off.

https://about.gitlab.com/terms/#subscription

Any thoughts will be appreciated