NOTE: These are not criticisms so much as release 1.1 (or even 2.0) use cases....
The categories are too broad (and, for some use cases, the 21 days is rather long, but I get it...).
Re categories: While I am interested in science, quote unquote, I am orders of magnitude more interested in physics than biology, with the exception of biochemistry as it relates to evolutionary theory, and within physics I am at least an order of magnitude more interested in QM (two orders, if you can nail it down to relational QM) than I am in astrophysics, and my interest in astronomy is even lower, and my interest in exoplanets is mostly nil.
And that's my personal interest stuff. Professionally, I am interested in JS specifically, so that would be programming, but I really have zero interest in Fortran or COBOL or Haskell, unless somehow those relate directly to things I can or might want to do in JS (full stack React FE, node BE).
The timeframe side relates to that professional side as well: That JS focus will change at some point (startup life) and by the time 21 days have passed, well, I'll be about 2.5 weeks behind, eh?