I don’t understand the point of this. Your concern is that most miners will choose centralized transaction selection, so you want to force all miners in a decentralized pool to use the transactions selected by other miners? How does that not make the problem worse?
Here’s an example: assume today that 99% of all transaction selection is performed by X. That means there will still be about one block a day that includes indepedently selected transactions—and anyone paying a high enough feerate can almost guarantee that their transaction will be included in that block (provided the tx is valid and policy acceptable). Fees are a good solution to weak censorship: it allows the people being censored to take direct action and buy their way out of their problem.
But if 100% of miners use your described pool, they’ll have to mine any transaction that was previously included in a share before they can mine any transactions of their own choosing. So whoever can include a tx in a share first gets to decide the transaction selection for every other miner in a pool. That creates a strong incentive to be the person who can first include a tx in a share: they can collect out-of-band fees that nobody else can collect and they can perfectly censor unwanted transactions by including junk transactions instead.