> I would ideally like to use kill-the-newsletter for that but I had trouble with delivery with substack.

Substack does indeed seem to have kill-the-newsletter banned, but (at least for free posts) it actually provides an RSS feed out of the box, so you should be able to just chuck the address of the blog’s home page into your RSS reader and have it figure things out for you. I haven’t seen this capability advertised anywhere (and the days of the RSS icon in your address bar are sadly long gone), but it does exist.

(Incidentally, Buttondown also has RSS feeds built in.)