Using a custom firmware, it’s possible to use the RTC inside the stm32/keyboard with only the 18650 to keep RTC running (tested this feature the last week).
I didn’t want to mess things up with rough soldering and batteries everywhere…
(…so I messed up the code instead :3)
But yes, you have to manage the AXP power supply differently and code all the interfaces from the stm32 to the pico this way.