I've built https://uptimelog.app as a small side project to scratch my own itch. It's free for now, so I don't have much feedback on how to build a business around it yet. However, I do think MacOS development is definitely a niche in itself, which can be a good thing because you have a user base that needs software for a specific platform.

I initially used Go for the backend and web as the frontend, so it essentially works as a local web server serving a web page in the browser. However, I'm now rewriting it in Qt (C++) to make it easier to maintain and to make it into a native application instead of a local web app.