I made myself a mail alias (on fastmail) that autoarchives into a folder and sends to an address that kill-the-newsletter.com/ created for me, which converts to an RSS feed.

(n.b. "Newsletter name" on "Kill the Newsletter!" just becomes the feed name, doesn't have to match anything.)

I changed my e-mail-for-substack to that address, and have all my substack subscriptions (free or paid) in my RSS reader, in a single feed and, thank goodness, out of my inbox.

Recommended.