Rack and fenders. I run the rack next to the dropout and the fenders outside. That way the bolt doesn't (and cannot) flex as long as it is snugged up tight. I put a washer under the bolt head outside the fenders so the bolt has no tendency to spread the fender stay eyes.

Done this way, there should be no noticeable flex until the bolt breaks. Threading for 6 m,m will raise that level, but with good bolts snugged properly, 5 mm bolts should do. An even better way to do this would be to run the bolts from the inside, ie heads toward the wheel. You could then snug the nut against the rack so you have a minimally short bolt holding the rack in place. But do this with a bolt with cm of extra length. Now fit the fenders on the bolt and use an second nut. I've done this in the past and it works very well. Use lock nut or lockwashers. You don't want the bolt loosening into your chain or cog. Plus is the ease of removing fenders while leaving the rack on.

Ben