Yeah, the issue was with Windows doing funky things with GRUB during an update. GRUB is a popular bootloader, which detects which OSes you have installed, and presents them in a menu. But Windows has been setting Windows Bootloader to run instead of GRUB when it updates. And Windows’ Bootloader doesn’t automatically detect Linux installs. If you use your BIOS to choose your OS (instead of using GRUB) you’re fine.
Even then, the fix is relatively simple in most cases. It’s just running a command in Windows’ Command Prompt, (the specific command is a little bit different depending on your specific distro) to re-enable GRUB after the update disables it.
Yeah, it’s a toss-up between the Nvidia Shield and the Apple TV 4K. Go with Nvidia if you want to be able to load your own custom launcher. Go with Apple TV 4K if you want ease of setup. Either one will be a workhorse of a device, both have robust app stores to run whatever streaming services you prefer, and both will run whatever custom media you want to throw at it via Plex, Kodi, or VLC’s network play.