Looking at the hitch owners manual - you are stuck using metric hardware due to the construction of the hitch. Stay with a class 10.9 bolt. Do not lower the bolt class to 8.8.
I am not a fan of a bolt with a lock washer/flat washer stack such as how Curt has called out on the leg to main body connection. From experience -this combination won't stay torqued properly due to the movement forces in play during operation.
I would consider going to flange head bolts, eliminate the washers and use high strength thread locker to avoid the periodic re-torque of the bolts. This would depend on whether or not I could get a flange bolt with proper threaded/unthreaded dimensions that mimic the original bolt. The use of washers does reduce the potential to gall the finish but will require the periodic re-torque due to the potential for movement with a minor loosening of the bolt torque.
Using high strength thread locker will require heat to break the bond of the thread locker for disassembly or re-torque.
Hopefully the OP has a large enough torque wrench for the required torque. Proper torque values go a long way towards having an assembly stay assembled properly.
