I see. It seems what I was hoping for was for a Jellyfin "streaming library" but that needs handling from their side.
I tested the setup nevertheless. I used --volname \\server\share to mount the remote with --network-mode. In network mode the explorer doesn't create thumbnails and listing directories was faster.
Jellyfin was able to create the library scanning all my directories. But loading photos was very slow. I suspect that was because Jellyfin was forcing rclone to download all the pictures. I found inside the cache some photos I didn't try to open so most probably 203GB was queued for download.
It was fun though. I had no idea about rclone and I am curious to see if I can play with it in another project, maybe setting it up in a Pi. Thanks for the help @asdffdsa