Maybe it's just me being super confused, but why would you need it to use a public folder?
For either Immich or Nextcloud, there are already software designed to (securely) host, provide and share data explicitly (and not share their guts to a public share over the LAN)
Wouldn't it be counter-productive to have them use a public share?
It makes way more sense for torrent clients, music libraries and the such, than for file hosting software that usually has to enforce user separation and priviledges in a way that the public SMB share won't reflect at all.