I’ve already answered most of these.
One time sync is out of scope for a continuous sync tool. Just use rsync.
Folder expiration is sort of a moot feature that feels like “and a kitchen sink” for a continuous sync tool.
Password for sync is also moot as by default you have to manually approve the request to sync something, already giving you the control. Trust is mostly based on devices not folders, so if you trust a device, you presumably should trust its folders.
Syncthing is not a public sharing tool, it’s for your own devices. Perhaps you are trying to fit it to a scenario it’s not made for.