I think you have a valid question, I just don’t see a better solution.
I’ve had plastic phones become unbearably hot to the touch as well as metal passively cooled devices. The purse/bag scenario is more likely to be an issue than the pocket as the pocket should notice the same level of heat buildup, the time to get there would be the difference I would expect. A purse/bag however is more likely to get to that unbearably hot level.
I really think the conductivity of the case/chassis will primarily affect time to temperature (both high and low) but not necessarily the maximum temperature.
I think the disadvantage of metal heating up in your pocket faster is balanced by it cooling faster once removed. As the Librem 5 is relatively high in mass as compared to other devices I’m hoping for a slower thermal increase/decrease as compared to modern phones, but that part is just hope not based on anything other than optimism.