The QB-35US3-6G has 2 connections for data transfer: USB3 and eSATA.
The DS720+ has the same connections.
I do not own a Synology DS but a similar model from QNAP. In my case, it is connected via eSATA and the 4x10TB discs installed in the device work without any problems. You can share them as a share
I would now expect this to work the same way with the DS720+.

Yes, that should work. I had this device connected to my DS1522+ via eSATA for a while. It then behaves like an external HDD.