I note that on https://apps.nextcloud.com/apps/documentserver_community it states:
The community document server is designed to make it easy to get OnlyOffice running in a Nextcloud instance without the need to setup an external document server, the community document server does not support all features of the official OnlyOffice document server and does not provide the same performance and scalability. (bold added)
And I'm guessing that the Cloudron app at https://cloudron.io/store/com.onlyoffice.coudronapp.html installs the full version? Is that right @nebulon @girish ?
So the method described by @gloes (thanks!) works, but perhaps does not have as the same features, performance and scalability?
So the current method of installing both Nextcloud and ONLYOFFICE Cloudron apps may still be best?