So here's a proposed way to set something up:
Your broadcasts all have an option in the left menu to Share the broadcast. Click that Share button:
When you click Share, it will open a new page with your broadcast at it's own url. Like this one, here's one of mine: https://ckarchive.com/b/mvu7h5hd73e335
From here, you can take this URL and put it on a Private page of your website that only your paid newsletter subscribers have access to. Say, Scottsbadasscomics.com/pastissues48 which is just a page with a running list of links to all your past Broadcasts. You'd update this page every time you send a Broadcast with a new comic.
To make it easy for your customers, you can add a link to Scottsbadasscomics.com/pastissues48 in the footer of all your paid newsletters.
Something like, "Read all the Past Issues of My Badass Comic over at Scottsbadasscomics.com/pastissues48"
I hope this helps! I don't know if there's a way to RSS something more automagically. Maybe there is. Hopefully this helps in the meantime!