Totally understandable! I'm a solo dev out of LA who's mostly consulted on various apps and sites over the years, and out of my own personal need I began working on the project earlier this year.
The project is indeed closed-source. Personally I've had experiences where I found the project moved forward towards the roadmap much faster - specifically when considering smaller-scale projects that are UI heavy and therefore take in many UI changes in parallel - when I instead implemented feature requests and focused most of my energy on listening to user feedback rather than reviewing code. UI polish can be quite tricky when it comes from many sources :)
That said, nothing is set in stone! If at some point there are enough feature requests to where I am the bottleneck, this is something I would revisit.
I'd be happy to hop on the Discord as well!
You can make the code open source without accepting pull requests or contributions from other people. You might still get pull requests or patches, but you can set expectations up front and then ignore/auto-close any contributions you get.
This is how SQLite is developed, for example.
More power to you! I bought Sublime Merge. I'd consider buying yours once it has a little more history. I've been using gg and really miss a side-by-side diff view. Maybe that makes less sense with jj though