Hi,
To help you with the _Host-Prefix, the following thread might help you. Please read from the linked post onward as there is some further explanation down below:
This solved the problem for me and others to get an A+.
So referring to your statement
It seems there is minimal benefit and perhaps some risk too from the need to move Nextcloud to the web root index.
As you can see in the link above, moving your installation to root is not necessary.
Best regards and good luck!