@fla I did it. It works well ! In fact I flashed the entire FP Open OS image before installing Ubuntu Touch, because with Lineage OS making calls was becoming trickier and trickier and I suspect the gap between the firmware and the image is the culprit.
But yes, it works well : I can call reliably, the battery life is longer (but I'm not a heavy user, so does it really count ?), and random reboots are really rare (like, once in six months or so). By the way do you have any news on Android 9 for FP2 ?