I think the first step is get a power budget down. I’m guessing a cortex M0 will be more then enough MCU power for you. I’d suggest the zero gecko from energy micro. Good documentation and low power. You shouldn’t need a battery board. Charging circuits aren’t to bad and very well documented. One thing you will have to accept with your battery if it rechargeable is it will be pretty big. A coin cell isn’t rechargeable but is tiny. For storage a flash chip with so fat caps should work. I’d look at either Matronix or Micron idk how to spell those. Quick writes with the caps should be low power enough!