I encountered the same situation a few years ago. I did not do a mileage run. I traveled around CNY the next year and ended paying K fare instead as I could not use seat guarantee. I still did not regret it. This depends more on the traveler.

Currently, long haul J cabins often get full before Y. Even as Diamond, the chance of op-up from Y to J is low, but from J to F is high. On mileage award upgrades, the chance of getting should be higher for Gold compared with Silver.