sorry for the late reply, https://documentation.immich.app/docs/features/read-only-gallery is how you do it. There's no ui way AFAIK.
You can include external folders to just be indexex/Metadata and keep the original as is
How does images work with federation? If 1m users on a big server look at my images that I posted on a small server, is that small server hit with that traffic? Or is all servers caching the image?
I know this isn't what you asked for but it might be worth considering at least (it's kind of what I've done). Immich is a photo app under very heavy development and have face recognition. They also allow you to import an external library as read only. This means that you should be able to mount your Synogy Photos library.
Might be worth considering at least.
Tried to read up on it and it semester like you have to enable it per conversation.
The poop-knife.
I actually find traefik rather nice to work with. I have a few Middleware chains set up, expose service using labels and add the chains to make sure I get the appropriate settings.
I would look at this https://youtu.be/uaixCKTaqY0 in regards to nginx proxy manager. It might not apply to you but worth knowing at least.
Atleast we got beens. Can't ve the darkest timeline then, right?