>Carrying 20,000 yen in cash is easy when it's 2x 10,000 notes, but when it's a mix including coins, it's a pain.

Carry a coinpurse. I use an old faux-suede Samsung camera pouch.[1] Plus it's fun when someone is running an errand for you, like getting you a snack from the konbini across the street, and you plop your coinpurse on the desk with a thud, then pour out a handful of 500-yen coins and say "this should cover it", like a feudal lord.

[1] kinda like this: https://www.amazon.ie/DFVmobile-Samsung-Galaxy-Camera-Closur...