Just the bolt under the front of the seat/back of the tank is all you need to undo. The strap & "buckle" at the front just hooks onto the tank - it all comes off as one piece.
Looks like you just need to remove that bolt that it's in the bottom of the tank securing the strap. Looks allen. Once you remove that the strap should be free. I don't think you need to remove the tank, just the seat