As you have found, there is no strong consensus recommendation for imaging after mastectomy with or without reconstruction.
A mammogram is probably the simplest way to image the new breast but you would have to discuss with your local radiologists regarding their experience with flap imaging. If they are very used to imaging flap reconstructed breasts, they will likely be able to read a mammogram as a means of surveillance for you. MRI is the most detailed way to look at the chest wall and all surrounding tissues, the downside is that it is so detailed that often things that turn out to be nothing stand out and can lead to unnecessary worry or biopsy.
So, physical exam remains the overriding recommendation but each case is different in terms of the risk for return of disease and so visiting with your local team and discussing a plan individualized for you is the best recipe.
Hope this helps,