Probably easier to think 'what's a thing to post that would be interesting to HN users' rather than 'what are the precise conditions for maximum posting frequency'. 'We're iterating very fast and have some new features' is usually not interesting enough to get around the 'about once a year for reposts in Show HN' standard.

Of course, you can, as you point out, post outside Show HN but the same applies - the promotional effect has to be a consequence of interestingness so if you focus on interestingness, you get both and everyone wins.