My dad used to say “Why did the man keep pounding his head against the wall? Because it felt so good when he stopped.” This seems to be a similar situation.
By saying “besides reducing transmitter power” you drastically limit the options. It’s obvious that the duplexer you continue to use cannot withstand operation as you wish. Your choices are really the following:
Reduce the power (you might be surprised how little that affects the range),
Buy a better duplexer,
Contact the customer service department for the duplexer you continue to use and subscribe to the “insert of the month/week/whatever club”,
Consult with a technical ceramics company to see if they can make you some custom ceramic inserts. They would withstand the heat, but eventually the heat might affect a nearby portion of the duplexer, shifting the failure elsewhere. Yes, that would cost some money, but so does repeatedly buying a duplexer that doesn’t operate at the level at which you have chosen to operate.
It’s obvious (thricely reinforced) that the duplexer in question simply isn’t designed for what you’re trying to do at the power output you insist on using, even if the manual or specifications differ. Saying “but it works so well while it works” simply doesn’t make good sense except as a temporary installation while researching a permanent solution.
Regarding choice #3 above, if you cannot contact customer service for the duplexer you like, that’s another reason to switch.
I wish you well in your quixotic quest. Please do let us know what you eventually decide to do.