If I log into any of my devices running IOS-XE ranging from 16.3.6 to 17.6.4, typing the "?" after the file prompt shows "on-reboot" with the description "Delay effects until RP reboot", which I find to be consistent with my description of "on-reboot...

The keyword "on-reboot" is correct syntax because it's telling the switch to install the new software "on the next reboot" rather than now. Nothing is actually installed yet, it's just pre-staged. The installation actually occurs "on-reboot".