I'm having a pretty good career in optimization and OR, even in the corporate sector...

VRPs are extremely complex MIPs so usually heuristics are used. So ortools is probably the best option in this case for cost.

CBC and Symphony for open-source MILP, Gurobi for commercial MILP. OR-Tools for some VRP work. I use R (ompr package) to interface to CBC, Symphony, and Gurobi. HiGHS is looking promising.

Honestly that constraint is pretty straightforward to formulate as a MILP. Four machines, each has a starting and ending time as decision variables. Total duration by machine <= 100 hours, and add non-overlapping constraints between the machines. Each machine's start time >= 0, each machine's end time <= 168 hours.

I'm doing almost exactly this right now on a client project (I consult in supply chain optimization)