I don't know a ton about Substack's under the hood features like payment so I'm not positive about this. But! Several people who used Substack switched to Buttondown and have had nothing but good things to say. I know that the service is lacking some features, but the developer stated they're going to shift the roadmap given the new interest in the service (in regards to comments specifically). There's a good comparison list here, and the BlueSky feed for the service is full of discussion, feature requests/whether they're feasible, etc.

I looked up ghost.io and my jaw on that comparison chart and my jaw hit the floor. I know it's expensive to send email but yikes!