Switch to tantalum if you have space, much better for battery powered as a decoupling cap, low leakage.

Choose a cap with voltage 3x to 4x of input voltage.

MLCC or tantalum caps ?

You can also go for the TI Bq25570 which has support to solar Panels, Thermal and Piezo Electric Generators and available to buy everywhere !

Hi
According to wiki, using authentication prevent from delayed replay attack, but what about using only rfm69 hardware encryption, Isn't safe ?

Regards

The problem is within the bpi SPI implementaion.
I once tried almost everything to make it work but no success.
I advice you to switched to a serial gateway as a more robust installation.