It's been more or less four weeks and I got more than 100 subscribers.
The only strategy I'm using is to re-publish my posts in various platforms like dev.to, medium, and in Reddit. One of the main sources of traffic comes from Reddit.
And my open rate is more or less 60% now.
I'm planning to write consistently and actively searching for new sources to re-publish my articles.
Another thing I have in mind is to start a blog around my newsletter. Because Substack archives are not very SEO friendly and I think starting a blog will pay off in the future for sure.