First of all, thanks for your answer Koepel!
The sensor will be somehow protected against low temp, we will be in storing it in a box with large border, and we will try to limit the air exchange (even if we still need some for pressure balance :) ). We also plan to heat the box internally. Not sure how much we will be able to balance with the -55 degrees C from outside, but we try to keep the box as warm as possible.
At an altitude of 30km, I'm unsure what would be the pressure exactly, but from what I read, it was about 10mbar as well. We pulled data from a site and draw a graph about it: https://docs.google.com/spreadsheets/d/1vN6A6cp5ad9ABYSzyYsBeURFJsVaMY-GcVkaKvdFCo8/edit?usp=sharing
We checked already some other sites, and especially some other articles about people sharing their experience around weather balloon, but we also try to learn from our own investigations about each point, and not just mimic an already existing experience. :) We also are limiting the budget for each electronic piece. We know there are solutions which are more expensive (around $100 or more), but we wanted to know if there was any other cheaper solution already existing.