Appoxo

Developer ≠ Management/C-Suite forcing implementation
(Does not apply to small-scale indie devs)

Wheres the fun in that? ;)

Yeah in that use case syncthing fits more than immich.
In theory you probably could use immich.
During a phone replacement you pause immich sync, use syncthing to transfer the pictures and then enable immich sync again.

I believe immich should be able to detect those "new" pictures as duplicates and not upload it again.

In theory you can do that.
It just doesnt redownload to (usually) DCIM.
But say you upload your DCIM folder to immich, they are both on-device and on your server.
Until you delete them on the phone.

Neat!

I am already using a selective sync for my emulator save data between my NAS, PC and steamdeck in a triangle sync and on top another sync for the data I generate on my phone.

Doesnt work during an RMA.
After that: Yeah, sure.

And how much does Apple like that?

They explicitly mention in the deployment steps to pin the release number.
After that it's just a case of reading the patch notes and looking out for breaking changes.

And obviously keeping backups :)

Didnt have any issue so far with immich.
A case of not reading the release notes? (no offense just asking)
And tbf, they say they arent fully done for now. Essentially a very public beta.

Any suggestions on how to optimize it? :)
Using it on my steamdeck and on-demand on my phone.

I wouldnt use immich like that.
You backup your images to immich.
Once there, you keep them there primarily and stream what you need (redownloading like it works on Google photos doesnt work like that as it stores the pictures in it's own folder).
At least that's how I use it.

Yes.
It can also sync across the internet if you have it set up appropriately.

Very neat and they have recently implemented (chrome) casting capability into the app.