Wiki disagrees but in UK I was always taught (apprenticeship, college and uni) fully threaded is a screw and partially threaded is a bolt.
One major cause of bolt fatigue failure is faces that aren’t flat / square. So thick paint, hacked off bits of spacer tube or heaps of washers can all cause problems. Poor quality plated 12.9 bolts can also fail due to hydrogen embrittlment.
So quality black 12.9 socket head cap screws / bolts with a machined spacer will be the most reliable option (which is pretty much what Salsa is selling in that link ^). Spacers – a few minutes work for a friendly guy with a lathe. Pretty much any local engineering supplies place will sell you bolts in small quantity over the counter.
Not critical for this application but on fatigue rigs at work I’ve generally found Holo-Krome to be the best brand.