the thinking is fine and as the other response says, SnapMirror to tape for volume snapmirror will be fine - we can look at LREP is you are using qtree SnapMirror.
the easiest option though if you can do it, is the suggestion of another FAS - if you can sit that next to it and just SnapMirror everything out (as opposed to NDMP copy) and then just hold a copy there or transfer to new provider whatever works.