I advise you to do the traditional way of keeping up the private keys and nothing beats better than writing it off on a piece of paper, a cardboard. While the plan sounds good but if the long term is what you're planning for, although cardboards and papers might crumple over the period of time but it's much better than keeping it on a device.

You'll never know what might happen for the next years to come. But as you've said, this is a back up so please make sure that you've got strong back ups where you'd write your private keys because we have to anticipate the unexpected thing to come. And with that, there can be a lot of things that might come and someone might steal that device compared to a paper or card board, no one gets interested with that for stealing unless that thief knows what's written on it.