Hi @pierrot10 and others,
Sorry, I was called away unexpectedly...
I've had no experience using nails other than testing the gyprock ones in the link I sent.
I think your idea of 9 pairs spread across the wall is excellent as the key data you seek is the moisture source and how it progress across the wall.
You can just connect the 9 sensors to one of those analog switch modules and the Feather and start logging data. You biggest problem will probably working out the distance between the 2 nails. Once it setup (in summer), you can make sure they all read the same value by a software 'calibration value' for each pair.
I'd also suggest putting a newer BMP680 device in the room, perhaps 3" to 6 " away from the wall, just to have a temp, humidity and barometric pressure. I don't know exactly what it my tell you, but one thing I have learned is 'data is like money', there is no such thing as too much! (especially when you only get one shot at it a year).
I think your chances of success look good as long as the distance between the two nails give you a sufficient data range.
Good luck, and I look forward to reading how it turns out.
Paul