I ended up getting this boutique charger shipped from Australia (which took more than a month because Australia Post still uses camels): https://www.tindie.com/products/rezwan/ml2032-coin-cell-battery-charger/

I'm still measuring how it performs, but so far it seems to only add a few weeks of fuel to the Voltcraft batteries - perhaps they're not compatible.

And only a few DAYS to the Maxwell ones. Could be an issue with those batteries, something with the charger or something about the devices I use them with. Still experimenting.

I've never had any such issues with AA and AAA rechargeable batteries which I've used since walkmans were still hip (not the same ones of course, but I seem to lose them faster than they die on me).