@qqlapraline The 5v and 3v3 pro-minis can both be modified to very similar low power consumption, Google low power pro-mini for a multitude of sites over several years detailing comparatives and examples. I had reason to dig into this as explained below:
With combined radio and RTC on a 3v3 pro-mini I was faced with a lack of available pins from the combination to deal with the 5v ultrasonics as well as providing voltage step-up. I tried a plethora of potential solutions before realising it was a losing battle with a single device and so many demands.
Adding a low power 5v pro-mini to the node to deal with the US and communicating via I2C to the 3v3 pro-mini ended up the most efficient solution for my case. It may seem counter intuitive to waste energy on two devices and two power sources, but not so, both sleep saving energy. In my own case I was going to end up with two devices and two power supplies anyway, but just two separate radio nodes, so there was no energy penalty.
The 5v pro-mini has one interrupt triggered by the 3v3's RTC for US readings, the second interrupt is to be enabled by the adjacent Gas Meter reed sensor, releasing the original dedicated node for other duties.
It is not completed and deployed yet as it is still sub-zero here and the ground is rock solid where the sensor head has to run.
