The common way to mount partition automatically is to use fstab. When using gnome-disks like you are, you are actually editing that file.
Under the hood, the system actually translates fstab for systemd, so there is also sense in making the configuration directly for it.
Now a few tricks:
Once a partition is mounted, you can access it through its mount folder. But if you want “direct link” in your file manager side-panel, you can either:
– use a folder in /media as mount point
– add x-gvfs-show as mount option
You can easily check if your automount configuration works without rebooting : mount -a
Lastly, a question :
Why are you trying to mount your extra partition as the root folder?