To be truly open in terms of privacy, open-source + auditable open releases is the way to go. I'd recommend a free extension with a paid plan which has additio

To be truly open in terms of privacy, open-source + auditable open releases is the way to go. I'd recommend a free extension with a paid plan which has additional features. Provide the basics in the free version and the really good stuff in the paid version. If source code is the same and it's just feature flagged based on a ping to a server to validate a license key, then it'd be indeed easy to copy the paid features in another extension. To a large extent you'd be relying on the web store moderation a copy to not be published and to tech-savvy users to not install it locally. However, that approach could ensure a wider reach of users so the share of the paying ones to be big enough in absolute numbers. Marketing the extension to non-developer audience would help reduce the chance of manual installation. The biggest difference would be that the developer audience could contribute to the open-source code and you'd be making money from those contributions. I don't think the income from that alone would be enough to support you in the short-term, but in the long-term with good expansion with other products, that could prove to be a very strong passive income.