/opt

UPDATE: Leaving the below for posterity's sake, but don't do it. Some Entware stuff won't work with the "standard" entware setup, so far "man" and "openssh" because they appear to be hard-coded to expect different configurations (such as UID 0 being "root", which at least on my AC86U, the GUI...

Chasing down why I can't install Entware man and man-db packages, I discovered there are actually two different ways to install Entware (https://github.com/Entware/Entware/wiki/Alternative-install-vs-standard). RMerlin's install script performs the standard installation, which seems appropriate...