I'd like to make the best possible note-taking experience out of it, that implies developing a web-app, mobile-apps and generally putting considerable resources into the project.

I think the best way to ensure all this will come to fruition is if the app/service itself was profitable, ideally I would like for it to pay for itself and also provide enough funding for future projects of mine.

I'd like the business to be defensible, but also avoid making the app closed-source. I'm not sure how I could avoid having the (probably simple) server reverse-engineered at some point in the future in order to avoid paying for an eventual subscription fee though.

What do you think is the best way to achieve profitability?

[1] https://github.com/fabiospampinato/notable