I really appreciate the attention to detail in the app. You asked about its viability as a business - I have no idea but I really hope there is a market for small design-focused indie apps like this.
One thing I’ve found a little contradictory. In one of your videos you mentioned that TaskTXT is not for long-term notes and todos, just a space for what you’re focusing on at the moment, and not be too concerned about losing data for example. To me the use of a plain-text format suggests the opposite, that this is something that is very easy to archive, search, backup, keep in a git repo, etc. And the opposite; if the purpose is not to plan out a long list of tasks with descriptions and notes before you start, then maybe there would be another design that makes it feel more like a quick throwaway timer that’s useful only for about as long as it’s running.
I think it’s a very clever design though so I would encourage you to think about how to support planning for slightly longer time horizons as well.