Not sure if you're aware but there's "Show More" button besides Reading List on start page, which I think shows all of the entries and stays that way.Yes, that is what I'm referring to, and then that allows you to keep the SideBar open on the Start Page and have it NOT be there when you navigate away to any other page. And yes, the Start Page does show the most recent items from my Reading List, but I tend to go back further than the 12 listings it shows.
I honestly DO like everything else about Safari 15. I like the extensions and I love the new Start Page and all of the available content and how it syncs across everywhere. That's great! But the hate of now having to go thru many steps to scroll through my Reading List just makes it all a wash for me. 😕
I find it strange that you like everything else though, personally I can't stand how broken tabs are and that we don't have a fallback to old style.