Thanks for your comments, much appreciated. The siding already has step flashing behind it, as mentioned above. I think that edge just needs to be cleaned up. My main concern is where the T&G beadboard soffit meets the roof. Those are some pretty big gaps there, which you can hopefully see from the pictures above. The concern there is not water getting in per se but wasps, mice, rats, birds, squirrels, etc—all of which are a problem here. I’m not sure caulking a gap that big—1” or so in some spots—is going to suffice.
I’m thinking now I’ll cut a straight line an inch off the roof, lay down two lines of geocel 2300 under a 1”x 2” furring strip halfway under the soffit and screwed down to follow the contours of the roof. Then cover that with a rabbeted piece of panel cap moulding (rabbeted side over the furring strip) nailed in to the soffit and the furring strip. If all goes well that should fully seal the gap and blend in well with the rest of the trim work.