You can use a tool called gibMacOS that can download the macOS 12 installer from Apple's Server without the need to enroll your system. Just install the "InstallAssistant.pkg" after download and then the macOS 12 Installer app bundle will be in your Applications directory. It's the exact same app bundle that the Software updater would download.

I somehow feel it could have something to do with HDCP. I also have that issue with my HP G2 Dock. Not only does it crash the machine, but regularly starts to stutter on the external screen (once every few seconds). If you then look into log console you will find lots of lines about stalled HDCP actions that come exactly when the stutter occurs. The latest firmware update of the G2 dock did indeed brought some HDCP fixes (mostly for HDCP 1.4 <> 2.2 switching). For now, I just connected the Monitor to the Thunderbolt downstream port on the G2 (which bypasses the DP Hub in the Dock) and have no such issues. There must be some problem with those DisplayPort Hubs and macOS. On Bootcamp there are no problems, but Windows also doesn't HDCP encypts your content if there's not reason for it. macOS does all the time if the devices support HDCP.