Updates not “taking”

I’ve got two old MBPs from 2011 and 2013 that I’m trying to update to 15.4. The updates show up, the process seems to start but after reboot the Macs are still reporting 15.3. Any ideas what I’m doing wrong here?

Bonus question

How the hell do I get rid of the “macOS installer” from my boot options?

Author: @LaSystemeSolaire