what happens if a specific node is aware of a certain set of transactions but doesn't broadcast them to other nodes (the network) and includes them in it's own hash.
Nothing bad happens, but the rest of the network will check to make sure the transactions are indeed valid once they receive the new block. If everything is valid then they accept that block as the new chaintip, and miners will start mining on top of it. In this way, miners can add their own transactions to blocks they find without broadcasting the transactions to the network first.
Wouldn't this automatically guarantee it will win the mining award?
No, the mining reward is claimable by any miner that finds a valid block. A block can be full of any transactions the miner would like (or none at all), but the exact transactions included will not alter the probability of a miner finding a valid block. If a miner fills a block with their own transactions that haven't been broadcast to the network, they will be at no advantage over a miner that is including only transactions that have been publicly broadcast.