For https://poach.vc/ and https://data.poach.vc/ I just emailed and LinkedIn DM-ed a ton of people. Since the newsletter features founders and investors, I also tag them each issue on Twitter, which is a small but evergreen source of exposure.
What we are currently doing: 1) Cold outreach to power users - to convert them into affiliates. 2) Cold outreach to individuals who have target ICP communities. 3) SEO for more long term (not for the first 500)