welcome to the forum @uninvitedguest 
Please read Nextcloud Collabora integration to understand the mechanics of the integration and follow troubleshooting steps.
The reason why almost everybody is using reverse proxy is because this is the only (good) way to host multiple applications on a single IP. Using custom ports like you do might look like a solution but doesn’t work in real life e.g. when company firewall blocks non-standard ports… the pressure from browser vendors on unencrypted applications is high so often this can be an issue as well (e.g. Password app only runs with TLS, WOPI/Collabora AFAIK too)
on the other side in a docker-compose setup it very simple to add reverse proxy see this post - and you immediately get lot of advantages - simple letsencrypt TLS certificate for all your applications, no need to bother with custom ports, in some cases even better security…