Out of curiosity I solved it with the concorde solver in the Neos server. In 58s its heuristic found a solution 0.037% away from optimal, and in 943s it found

Out of curiosity I solved it with the concorde solver in the Neos server. In 58s its heuristic found a solution 0.037% away from optimal, and in 943s it found and proved the optimal solution. (This is with 3GB of ram and 4 threads of an Intel Xeon E5-2698 @ 2.3GHz aka a 30yo algorithm on a 10 yo machine)