We will save our Duplicati backups (from windows stations) on a TrueNAS Core server. What protocol you suggest to connect Duplicati with TrueNAS.
I can’t speak from TrueNAS experience (anybody out there who can)?, but seeing TrueNAS protocols, and knowing what’s often available on Windows and Duplicati, I think less available ones are AFP, NFS, iSCSI, rsync and TFTP, which leaves SMB, SSH, and FTP. Authenticated SMB from Duplicati can be tricky to set up, and Windows may make it impossible for a service (some people have figured out workarounds for it).
FTP without encryption is insecure over the network (if that matters), and securing it with encryption needs some setup for certificates. SSH (SFTP) is a simpler setup. I can’t tell you which your system runs fastest.
Probably we will use SSH (SFTP). We have a lot of users and the administrative overhead must be minimized. The performance matter but the simplicity win.