newsletter or just a public changelog when i have 3 hours a week to write Newsletter

Neither, at 6 paying customers. 3 hours a week spent talking to the 90 free users will teach you more than 3 hours of writing to nobody, and it converts. Publishing is a lever you pull once you know exactly who you are talking to - right now you would be guessing at the audience and the message simultaneously.

Changelog first, and let the newsletter be a byproduct rather than a second job.

The mechanic that worked for me: write the changelog entry as part of shipping, one paragraph, in the PR description, and have it end up on the public page. Once a month, take the four or five entries, add two paragraphs of context at the top about what you are trying to do and what did not work, and send that as the email. The writing was already done - the only new work is the framing.

Weekly newsletters die because 3 hours a week is 3 hours in a good week and 20 minutes in a bad one, and one missed week turns into three. Monthly survives a bad month.

Whichever you choose, own the list. Even if it is a page with a form and a CSV of email addresses for the first six months. The single most valuable asset from two years of writing has been about 3,000 addresses I can reach without asking anyone's permission, and the platform where I originally found most of them has changed its distribution twice since.

Do both but merge the surfaces: the changelog page is the archive, each monthly email is one entry on it with a permalink. One thing to write, two places it lives, and the archive page picks up search traffic for feature names people are searching for.

The 3 hours will not be 3 hours. Writing an issue is maybe 70 minutes of actual writing and then 90 minutes of second-guessing the subject line, fixing the layout in the editor and staring at the open rate afterwards. Budget for the staring.

The changelog also has the underrated property that nobody expects it to be interesting, so you can never disappoint anyone with it.