love it!
about your unsolved questions:
To keep people coming back, I think an off-site notification channel for (opt-in I guess) nagging notifications would be something I like. Ideas for this would be serviceWorker notifications in modern browsers, email notifications (meh) or maybe automating @thestreakclub to tweet at a few people with a fun reminder (a bit like @indiewhipbot).
For the second part, a unified hashtag might be useful, when I do things I try to tag @thestreakclub but using a handle like a hashtag is a bit clumsy sometimes and people use it less I would imagine. Also I guess if you thought of a smart way to promo streak.club from itch.io via something like the itch jam that could work wonders (though streak.club probably fits artists best, and jammers are likely going to be programmers in majority).
Also - would you be up for a weekly loop streak again some time soon? I've been sporadically doing a few over the last weeks but streaking and non-solo is more fun, plus I desperately need some inspiration :D