IIRC the first release was supposed to be early June. Have not checked. No tutorial yet, maybe I should write one. But you can basically follow ANY OnlyOffice install procedure talking about Docker image, and replace the onlyoffice with euro-office, and it should just work.
https://github.com/Euro-Office/DocumentServer -> Run this to get the server going.
https://github.com/Euro-Office/eurooffice-nextcloud -> Run this to install it into Nextcloud.
So much this. And if OP uses only OpnSense it can be done in there directly. Just so much easier than hairpin NAT and all the other stuff.