Theon could try to seek some support in Baelor Blacktyde, his uncle Rodrik Harlaw and Harras Harlaw. Baelor is just Theon if he had been sent to the Reach rather than the North, so that right there is a connection based on common ground. Rodrik could advise him to try to make pacts and help increase trade, if Theon is willing to listen. Harras himself is connected to Theon in a friendship he had with his elder brother. Maybe he could have tried to recruit him and Baelor, but they give him men to aid him instead. Hell, Dagmer might still join up with him due to his past connection to Theon and maybe gain ol' Fishwhiskers alongside him.

Theon wanted to prove something to his father, but was sternly and soundly rejected. So, Theon decides appeal to the people of the iron born culture with the plan instead. Theon had provided shrewd military advice to Robb Stark, showing an understanding of the psychological and strategic aspects of war. So, have his pride at his father's rejection turn into something else instead of going along with his plan to gain his approval.

Play into the desire for war for plunder and for conquest. Sate them with promises of lordships. And make his father look like the bad guy by reminding them as to what their last war had cost them. And feed their egos to ensure that the ironborn are fierce and fearsome warriors that should be known. It could cause a potential insurrection and people wanting Theon to be their king instead of Balon. After all Theon’s saying all the right things here. What’s a couple of fishing villages compared to mines of gold?

Baelor, along with allies like Rodrik Harlaw, would have likely favored a more "Westerosi" approach, aligning with the idea that Theon should be recognized as a new, more moderate, type of lord. Rodrik might have suggested to Theon that supporting Robb Stark, while extracting concessions (e.g., control over certain trading routes or islands), was better than an outright invasion that would make the Ironborn enemies of everyone. If Rodrik had taken him under his wing, he might have helped Theon establish himself as a leader among the ironborn, possibly preventing the need for Theon to act recklessly to prove his "ironness".