London Review of Books. It’s a print magazine mirrored on its website, but do yourself a favor and leave your phone beyond reach while trying to read long-form. The internet has ruined us all.
I see it's published twice a month. I also see that it has an RSS feed that doesn't seem to have an obvious link on the front page: https://lrb.co.uk/feeds/rss
It's more than book reviews, also social and political commentary.
And often the book reviews will be nominally about 3-4 related books, and give really interesting in-depth takes on historical events, social movements, etc.
I see it's published twice a month. I also see that it has an RSS feed that doesn't seem to have an obvious link on the front page: https://lrb.co.uk/feeds/rss