Can a miner just introduce a block full of valid predetermined transactions in the blockchain and get rewarded?
This will not work, because a block's content includes the previous block's hash which means that a valid block can only be mined after the previous block is known.

The dummy transactions would not yield a reward to the miner, because the transaction fees are just going from the miner's left pocket to the miner's right pocket. The miner will only gain the newly minted coins, which they would have collected anyway, even with an empty block that included no transactions except for the coinbase transaction. In fact, the miner is suffering an opportunity cost, because they could have collected fees from other transactions instead.