What I've found to be more successful than anything else I've tried is to give staff some choices:
1 - leave the device and pick it up in the fall
2 - leave the device and pick it up a week or two after school gets out (however long you need to do the imaging)
3 - For staff who are taking classes, workshops, etc. right after school or traveling elsewhere for the summer and have to have the laptop in the week or two after school the must turn it in for imaging before school gets out. We set up a specific time period during which they schedule a block when their device can be imaged and returned to them. They can borrow a "spare" device for an hour or two if they can't get by without having something.
4 - Switch to a ChromeBook so that no re-imaging is needed
While backing up bookmarks, music, and other stuff is inconvenient, if they're doing work in GSfE backing up files shouldn't be much of an issue. If it needs to be done anyway the top 3 options meet everyone's needs.
I usually share some documents like the attached to help people prepare for re-imaging.