New schematic update:
The device is comprised of 4 parts (from top left to lower right): an OLED screen for display, an RFM95 LoRa module, an ATMEGA 328P microcontroller connected to three buttons and two small LEDs, and a power supply regulated to 3.3v
New board design: double-sided, with battery parts on the back side and everything else at the front. Changed SMT buttons to bigger through-hole buttons.
Milled board + fabrication:
Code available here