> I don't want personal care items that have internal batteries at all, because they'll eventually die on me while the device itself (brush heads notwithstanding) is otherwise perfectly functional. I'd much rather keep rechargeable AA(A)s on hand for that kind of stuff.*

I thought the same, but that'll happen with devices that use AA batteries too.

I was using a cheap Panasonic EW-DJ10 water flosser that uses AA batteries, but the battery compartment contacts would corrode after some time.

I then switched to one of those China USB-C water flossers, but I think the built-in lithium battery's nearing the end of its life since it'll cut off randomly now. I also used to worry about charging it, since the USB-C port would need to be dried out before I plugged the charging cable in.

No idea what's a happy medium for electronics that need to work near water.