In my opinion, 7 weeks is too early for an elliptical machine. Until the fusion level stiffens (at least 3-4 months), no up and down loading should be considered. A stationary bicycle and simple walking with hiking poles and pool walking should be the order of the day. Even though you don’t have a disc, you have a spacer (“stone”) wedged between two bony vertebra held together by thin ligaments and screws in the back of the spine (hopefully) which cannot withstand up and down pounding. I would back off.