Ok so little update…
During the night the phone was on WiFi, with data services enabled (but theoretically disconnected, right?). Phone rebooted at some point.
In the morning I disabled data services: both SIMs where in and in roaming, internet only on WiFi. Phone didn’t reboot.
Then I checked the APN settings and found out that a new APN appeared. I selected the old one and reactivated data services a couple of hours ago and since then no reboot (yet?).
I’ll update if the situation evolves. Thanks for the support everyone 