I had the same experience with Amazon a couple of months ago. Doing a search for the 5600X returned no results week after week, but then I just happened to follow a link that I found elsewhere to the "hidden" product page and it was there, in stock, for the $299 MSRP. An hour later, it was out of stock. I felt really fortunate. I, too, didn't have any of my other parts yet, so my 5600X was sitting in its box on the shelf for about two weeks.
Enjoy. I really like mine. I've had it installed for over a month and am still tweaking to get the best overclock. Don't neglect PBO and Curve Optimizer. I've found that
https://www.reddit.com/r/Amd/ is a good resource.
Sorry that I'm a few days late. I've heard good things about all three MSI lines, but went with the ASUS ROG Strix B550-F Gaming (WiFi), myself. A few things that I like are its variety of USB ports in back (more than any of the MSI boards that I looked at), the all-black design, included heatsinks for both m.2 bays, the AC1220 audio, 2.5G ethernet, the WiFi (w/ Bluetooth) option (there's also one without) and ASUS's timely BIOS updates. It'd probably be a good match for that case that you picked out because of the all-black look and RGB support, but you probably couldn't go wrong with the MSI boards, either.