I wrote a weekly newsletter about React at https://maxrozen.com, got to about 1600 subscribers over 9 months or so.
I eventually pivoted my attention to building software again, but I write a (more niche, investor-like) newsletter for about 100 people interested in my product's progress.
The biggest tip I have is consistency. Folks like when you publish on a schedule.
I write a newsletter called 10+1 Things, having close to 1000+ subs.
You can read it here: https://rishikesh.substack.com/
I write a daily curated newsletter called SMMRY covering the best stories in tech, science, engineering and development.
As of now, it is close to reaching 100+ subs.
You can read it at https://smmry.tech/