Metric fasteners aren't graded as Grade 5, Grade 8, etc. The factory hitch bolts are very hard, I'd say at least the equivalent of Grade 8, but I don't remember what that's called in metric terms. Grade 10.9 maybe?
Basically, you want hardened bolts, not everyday-strength bolts. The factory bolts have flanged heads, which I have not been able to find an aftermarket source for, so you'll also need matching hardened washers.
The factory bolts also come with loc-tite in the threads. I put anti-sieze on top of that, which is easy because the thread locker is dry. If you can, put blue loc-tite in the threads a couple of days ahead and allow it to dry, then use anti-sieze when you assemble. If you can't loc-tite it ahead of time, skip the loc-tite and use anti-sieze.