I have had a reply to this issue on the Nextcloud Forums. The inability to adjust settings via the GUI, particularly in relation to wopi details, is a known issue.
The command line solution requires employing occ, Nextcloud’s own command line. Unfortunately, I am struggling to apply occ commands in Yunohost. I have found this forum topic [SOLVED] Using OCC command on Nextcloud and a blog article on the subject: OCC Nextcloud Command in Yunohost – WONKO.NET I have got in to occ, but am not able to get the config file to actually open. I have posted a full description of my struggles here: How do I point Nextcloud Office app at a differnte URL? - #3 by danceswithcats - office - Nextcloud community
I will update on my progress.