I’m not sure what ‘LEA’ is. I was thinking of Let’s Encrypt specifically but any other certificate authority should do.
As for your build, I want to reiterate that you should build up slowly and test all the features as you go. Don’t include code or fancy customizations that you do not understand. In this instance less is more. Once things are running you can backtrack, add to the server and tweak to your heart’s content.
If you encounter issues you can post your configuration files here giving a detailed description of where you are at in the build and I am sure that a reader will help with suggestions. On the whole the readers are quite astute and willing to help if the writer meets them halfway.
So to this end I would setup a fully functioning Seafile running on port 80 (HTTP) and then add HTTPS. Report back here and let us know your progress before adding OnlyOffice. I think you will meet with success.