there’s a setting for the radio strength between the halves. i‘ve cranked it, cuz i want the halves to be able to communicate through obstacles, but lowering it may improve battery life.

If you're using zmk, you should investigate the viability of a dongle for your setup: it's a third nice!nano (or other Dev board) that's directly connected to the computer (without battery) and serves as central side while the two parts of the actual keyboards are configured as peripheral.

Since central's energy consumption is massive compared to peripheral's, weeks or even months between charges in this topology isn't unheard of.