With great power comes great responsibility...

Yes ! I finally found this power issue and managed to solve it. Wiring was fine.

- First : Step up / step down module

In simple words RTFM (Read The Fine Manual) or at least translate chinese manual...

Those module need to have at least 2V between input and regulated output voltage. In the first attempt, my powerbank delivers 5Vdc and enter in a step up/down module and output a 5Vdc which is apparently bad for a raspberry pi. It rebooted permanently.

Change the input for a standard 2s 18650 battery pack 7.2Vdc and it works like a charm.

- Second : too smart powerbank module

This kind of module needs a "smart" device to communicate with and deliver needed power. Thanks to PD (Power Delivery) protocol or some kind.

Connecting a simple device such a usb lamp (+ & GND) will not work as you wish. With module I bought, you had to push the button on the powerbank and the lamp will light up for... 30 seconds. Too fast and totally useless for a rugged rpi computer. Holding the button for a bit longer and the "ON mode" is active for around 1 hour. Better but still useless.

So removing the powerbank module and replace it with a 2s BMS module and a step up/down module to be able to recharge the pack with something external (solar, cranking device or standard AC to DC source).

All module have been simulated and approved to work with lab bench.

Now waiting for items to be received.

Update with pictures soon.