Will LuCI mountpoints appear after system detect a disk? Now there is no such menu path.
I think so, since there is no specific "mount" or "disk" Luci package, i think if the "block-mount" package is found, the luci mount tab should show-up.
Sure sftp/ssh or ftp/s is for getting secure file access over internet, while smb/nfs are designed for LAN. Yet smb/nfs is often faster than sftp. The main reason you might want smb/nfs over sftp is that those behave nearly identically to a local filesystem. So you can start applications, lock files, stream movies normally, while sftp either wont do any of those or at least for streaming needs special mount tools like https://mountainduck.io otherwise it will always download the whole files.
From my experience sftp works best for very simple use cases, where you manually up/download files and than open/edit them separately.
While smb/nfs works like a normal filesystem and you can directly work with the files or open them from your applications.
The solution to circumvent the security issues for smb/nfs is to setup a VPN like wireguard than your shared network drives will just show-up as if they where local, if connected over the internet. Wireguard is so efficient that you should get the same or better speeds this way, compared to sftp while retaining most of the benefits.
Sure, avoid static ip's if privacy is a concern.
If you want to reach your router externally, the best option is to setup a VPN and buy a cheap domain you can setup as DDNS. As example namecheap.com works via the openwrt ddns service, so you can reach your system via name.