I think you could actually delete the teachers without much issue. A deleted user actually stays in the database but just has the deleted tag set to 1 instead of zero. So, if the teacher returned several years later, you could actually reinstate them directly in the database. (I discovered this when one my admins randomly deleted two students by mistake!) I would of course, recommend testing this first with a test teacher to verify that everything still works the same way but I suspect that it will.
Enrolment
Suspended users removal from cohort
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