What I want is to be able to drag and drop files in my remote server to and from my desktop as if it's an NFS/NAS. What's the best option for this that will fully saturate the link?
I don't know about saturating the link, Buy sshfs can mount remote dirs and then you can drag and drop
KDE/Dolphin has support for sftp built in. If you have sshd on your server you can just open/bookmark sftp://host/path/ as a folder and use it normally. I get around 0.9Gbit/s on a 1Gbit/s link.
CIFS/Samba has had a lot of effort put into it, so setup Tailscale and just use that, if you're after drag and drop.