The Photos app supports External Storage mounts just fine. (And it has since June 2021).
That thread is five years old.
No idea what the couple 2023 comments on it are about; thatâs kind of the problem and why the thread likely got closed: The mods have been cleaning out old dormant threads in the forum so that people donât pick them up later when the info in them is no longer accurate.
Unfortunately there are thousands so itâs not realistic to add clarifying comments to them all.
So this has been reported for 5 years and then this thread was just closed? External Storage - Linked to your Photoâs App
Thatâs not a report. Itâs a random forum post. Itâs neither a bug report nor enhancement idea â those are handled via GitHub.
Is it honestly not possible to use network storage in any form to link with Photos app? I mean honestly who runs Nextcloud on a 2TB drive?
Yes, you absolutely can.
And, well, plenty do to your second question too. 
I donât actually use the Nextcloud server itself to host ANY files, it is all linked via an NFS share. So why canât Photos do the same thing?
Then even that old thread was irrelevant. NFS shares are handled at the OS level. Theyâre not mounted via Nextcloudâs External Storage support. Nextcloud just sees them as local files.