Instead of 10 (or 15) small holes in your wall, how about six larger ones? In my case it's only four, but you'll get the idea... assuming of course I can get the photo to paste here.
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This is a hunk of quality 3/4" plywood, with four burly and long screws (lag bolts recommended) holding it into studs. 1" would have been better, but this is what I had. Filled the holes, trimmed it out, painted, and then mounted the hangers where I wanted them. I was able to go up a size from the fasteners that came with the hangers. Barely went through the holes, but the additional diameter was worth it.
Slotwall is another option, but maybe not if you're married.
Also, since you have basses with 'uneven' headstocks, you'll want to make sure you get hangers that swivel, otherwise the basses will hang funny.
Immediate edit to add that the type of hanger I have almost require what I've done here. There are three holes for fasteners, and there's no way to get all three into a stud. You'd have to be precise to get *two* into a stud. The two holes inline obviously makes this far easier.