What kinds of advice or ideas do you think would be helpful that's specific to mathematicians? Any (hypothetical) examples? Are there things that are currently 'painful' or that you expect to be, that you don't think your (hypothetical) workflow/productivity system addresses?

Possibly helpful: I use GitLab for all of my projects; not just software. I find GitLab issues work very well for all but the biggest/longest projects (and even then it's easy enough to split up a project into 'sub-projects' with separate GitLab issues). One reason I like GitLab is that it has a very nice Markdown dialect and it includes pretty good (for me) 'math' support. (My favorite part of it's Markdown is todo lists, i.e. checklists. I find that very useful for, first, outlining what I want to do, and then, later, recording that I've done all of those things.)