I get the desire here, but disappearing messages really just add a level of inconvenience without doing much to increase security. No matter how secure the system, one can always take screenshots, or worst case take a picture of the screen. (And for personal relationships as you describe, the difference between a signed message and a screenshot is going to be slim to nil.)

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It's just a good practice, works against 99% of people who never bother to take a screenshot.

If it's such an inconvenience, make it 1 month instead of 7 days. But if you need that much permanence, you need to start to question if email would not be the best medium for that specific conversion.