Although I do suffer from aching hands due to vibrations, I don’t really suffer from arm pump and I think this is due to my climbing. Therefore, I think climbing type exercises would be good as there’s an obvious crossover where bikers and climbers try to delay “getting pumped”.
Things I’ve used include; Power Putty, Gyro-balls, Black Diamond Doughnut (a rubber ring you squeeze), reverse curls (arm and wrist), squeezing a punctured tennis ball, and “Hectar the forearm correcter” which I made from a rolling pin, sash window weight and length of cord. Wind the weight up in a very slow, controlled manner (elbows locked into your side), then unwind and wind up in the other direction. This is the best exercise I’ve found.
HTH
Kev