I know it's a joke, but it got me wondering: What kind of treatment do you need nowadays to actually safely destroy flash storage? I doubt a single smash from a sledgehammer/boot has a good chance of destroying SIM and internal storage and µSD card such that forensic analysis can't recover its data. Thermite?

Take a look at this talk from DEF CON 23 on what it takes to safely destroy HDDs (spoiler: even explosions don't quite cut it): https://www.youtube.com/watch?v=-bpX8YvNg6Y

If you have full disk encryption (that you can rely on and no other security issues to deal with), then just erasing it by "throwing away the encryption key" is easier than physical destruction. Are there reasons why physical destruction would be preferred if these conditions are met?

A microwave oven works really well... Only takes (literally!) 1 second. OK, 2 if you want to be thorough.

It's flashy, but does it really destroy the charge in the cells?

Why make everything so difficult? So many options that it's hard to choose: EMP, solar storms, lightning, pyroclastic flows.. just stand right next to any cataclysmic cosmic event and you'll be taken care of.