Locks I'm thinking this might work quite well:-
https://tigrlock.com/product/
The wider one either standard or long - nothing will be perfect but this wouldn't interfere with any of my luggage and puts the weight on the frame, about 1kg. I might throw in a cheapo alarmed padlock with thin cable as well.
A titanium lock!!
The only time I ever worried about pannier contents was when I went into a caff and the bike was out of sight. My old panniers were/are zip closing so I used a small padlock to lock the zip to the carrier and stop the opportunist thief. It may require some thought but you could probably devise a simple deterrent for any pannier system.