To get a starting page reasonably similar to the old main page, go to
https://boards.straightdope.com/c/main/6/l/unread
On the left is a decent-looking list of categories/forums, and It shows unread threads on the right.
Less alike if you are viewing on a “mobile” device. I made that switch for all devices since it hides that annoying, to me, line of avatars for the OP and key participants in the thread taking up a big chunk of my screen. In mobile view it simply shows three threads for each subforum and some data about thread numbers in that subforum. Mostly those three threads are taken up with stickied posts.
Fortunately, I only rarely used the old main page. The difference between mobile and the other view is worth mentioning, though.
Yeah, I don’t know if Discourse allows it but if that was the “default” view when coming here via the usual URL that might smooth the transition for some people.
(Edited to fix a brain-dead iOS autocorrect)
Thank you for the tip.