You wrote almost the exact blog post I was considering and use almost the same exact method that I use. Glad to see I’m not the only one doing it a certain way. One thing to know, if you turn on Multiple Inboxes, and don’t see anything different, you might need to go into Settings → Inbox then make sure you have “Default Inbox” selected for Inbox type and then uncheck all the Categories. Then save and you’ll be sent back to your inbox and will see the Multiple Inboxes.
I wish there was a way to use both the Categories and Multiple Inboxes but I can’t find a way to enable both.