Started with a clean VM, installed web server, Nextcloud, domain controller, created a user, uploaded some data into Nextcloud. Then followed my steps from above, did
config set fw_nginx service status enabled TCPPort 8081,8082 access green followed by
Logged into Nextcloud as admin, enabled the onlyoffice app, entered the Document Server URL as https://neth_fqdn:8082, and saved the settings–which went without issue.
Logged into Nextcloud as an ordinary user again and added a document. The above was the result. Next step is to try rebooting the VM and see if everything still works.