Please consider a 3.7V LiPo battery.
When the LiPo is connected to the battery pad, it can be charged from USB and the battery voltage can be read by the XIAO.
The XIAO itself consumes little current when it sleeps, but the display is likely to consume the most current.
What is connected to XIAO?
With a slight modification to the sketch, the sleep current can be reduced to 2.5uA by disconnecting the display from XIAO during sleep.