Like when miners are paid off chain to prioritize transactions? Where are the “rules” of Bitcoin mining?

Depends what rule it is. If it breaks consensus, then it’ll create a fork. If not, then it’s just a different implementation.

'Rules of Bitcoin' is a conflation of policy and consensus.

Mining pools are free to leave money on the table by selecting transactions.

People are free to spend money to put monkey JSONs in a transaction.

Both are valid behaviours, as the blocks by Ocean are accepted and the monkey JSONs are mined.