Dolphin now supports server-side copy over SMB and NFS shares.
You need to mount the share via the kernel module (cifs) for it to work.
Don’t use Dolphin’s built-in “Network” feature. Don’t use locations with smb:// or network:// in the address bar.
Mount the share via mount, fstab, or systemd-mount. You can also use the GUI program Smb4K, but it has its own bugs.
Add the share to your “Favorites” for quick access. If you mount it to /mnt/truenas/myshare, you can add this location to your Favorites.