[Question] Email, Newsletters, Socials

Hey there, I have an application I maintain that's starting to reach a point where I'd like to start advertising it actively.

Context

I have my own blog that I built, it's on a sub domain of the actual application. However, I find it really difficult to advertise this blog in the first place. I cross-post to Medium, where I have some following and viewership, but that doesn't necessarily get the user to my application, or even my blog.

Thus, I was planning to start trying to gather email addresses so that I could email out updates when I create a new post/feature/etc.

The process has been kind of overwhelming to be honest. But I figured I'd start with just a few questions. Feel free to plug any of your own stuff and I'll check it out:

Questions

Is something like Substack a viable way to email out updates (given I have my own app/domain/blog)? Or at this point should I be hand-rolling and sending out email on my own domain?
I have 300+ users in my application, is it okay to add their email to my subscriber list? Or is this against some sort of policy/laws/etc?
How do you promote the app/page/social that gets the potential user to sign up for the email in the first place? I have essentially 0 social media following outside of Medium (which I'm trying to get away from). And I don't use things like my Facebook for advertising. Do things like promoted twitter posts work?