Option to copy as different user
I would like to protect my NAS from ransomware, so my PC's user account only has read permissions to the NAS. But if Teracopy had an option to use credentials for the user with write permissions to the NAS (and those credentials were stored in such a way that ransomware was ignorant to them), that would allow me to have the convenience of read/write access to the NAS without the worry of ransomware being able to encrypt my files.
Can't you connect to the remote folder using mapped network drive with different credentials?
Use it, copy and disconnect mapped drive.
But as long as you access the remote share with the privileged credentials the storage will be vulnerable too as every access from whatever user that has the right credentials will have access. There is no "user" access, but credentials privileges. If your Windows "user" accesses with writing privileges (let's say another user and password to the remote share), then your Windows "user" will access everything while using those credentials.
You can only minimize the time connected by manually connecting with privileged credentials, copying and then immediately disconnecting from that credentials.
If you think of using a second user ON Windows to access the remote share and copying, then that second user should not have permissions to access the first user files so it could not copy/alter anything from the first user. If that second user has rights to access the first user files then you have a problem as there's no difference on using each user as they both have right to alter the files and ransomware has already rewritten the files.