Not to discourage you from building your own, but Joplin seems to already tick all or at least most of your boxes -- in fact, I was surprised that it was was not addressed in the first article.
Thank you for your suggestion. In fact, some people have already recommended Joplin to me, but it seems that it was a bit late. Where I am, not many people seem to be familiar with this tool. After being suggested, I tried using Joplin and it turned out to be a good tool. However, you know, I had already started developing Notas a few weeks before, and abandoning it is the saddest part. That's why I initiated this discussion, and you contributed to it.