Another thing you can try is calibrating the camera (e.g. in Agisoft Lens). It doesn't take long, but will speed up the processing and the alignment should work better.
But the large 'blank' areas in your model are results of insufficient coverage and camera calibration won't fix it.
With detailed models like yours, always use High quality point cloud reconstruction. Medium and even Ultra high will be worse (Ultra high generally produces more holes in the model, at least that's my impression).
Sorry for double post, I sent the previous one too soon