When I first tested Restic with Plink (2 years ago) I had many issues with it, mainly performance and stability. I ended up using OpenSSH in Windows directly for my SFTP connection, keys being stored in the admin’s .ssh folder. With recent Windows versions, it is even easier than before, as no “OpenSSH for Windows” external package is needed.
Just try “ssh” in an elevated command prompt and you’ll know if you have it. Make sure you generate your keys (
ssh-keygen -t rsa) and provide you public key to the server’s authorized_keys file. You can set host, username and ssh ports of the remote server in the ~/.ssh/config file if you like. Once you can ssh <yourserver.fqdn> without password in the prompt, restic will also be good.
Hope that helps.