[Richbot]

The other thing I was thinking about was a socket head shoulder bolt with a 90mm-ish shoulder, it'd use a smaller M10 nut but would be strong where it counts

For those whose bolts have broken, where is it breaking? I'm assuming at the nut but I could be wrong

That's a promising source, would just need washers since they're not flange bolts but I don't think there is such an animal as a galvalume flanged M12x100 body-fit partial thread bolt in 12.9 or ASME/SAE grade 9

Well, a 12.9 socket head cap screw seems like the best bet, that new zealand site isn't very informative and he's probably selling SHCS's. Yield stress is 60MPa higher on than the minimum tensile stress of the 10.9 bolt but not sure how brittle they are. I would really want to know more abuot the failure mode before I changed anything.