Immich probably uses full paths and expects to be hosted in the root. Hosting on a subdomain will be the easy choice. If you insist on the sub-path you can rewrite requests in nginx to remove part of the path before proxying to immich

With a static site generator you can host it for free with Cloudflare Pages.

If you were to use ZFS you really should consider ECC. Filesystem checksumming doesn't do you any good if it's calculated wrong because of mem error.