How do you build a following for a newly launched newsletter?

I recently launched a newsletter called The Ideas Digest here on Product Hunt and I am using Substack. Unlike others, I do not have any preexisting mailing list to import, I am starting from scratch. Furthermore, I am known in open source circles and front-end development, but not so much in the niche I've targeted with my newsletter. I submitted my newsletter to PH and received a couple of upvotes, as well as a couple of signups. However, I feel as though I could be doing more. Do I just need to give it time and let people naturally find my newsletter or are there ways I can grow my subscriber base without becoming disenfranchised and giving up? I don't want to be "that person" who continually spams their newsletter on sites like Hacker News. Maybe I need to start buying ads, or is that silly for such a young newsletter? The newsletter itself centres around a relevant topic to many who frequent this site, it's about ideas, discussing trends, identifying opportunities and does research into various topics for the reader. I want to give people the tools to discover new opportunities and maybe start something of their own. Just because I can't launch doesn't mean someone else can't.