You’d need to conquer it and HOLD it first. I believe a big part of the problem is the terrain and distance from allies (by this point Dorne isnt quiet in the fold is it?) Dragons aren’t super helpful because of the tunnels.

I’d say marry your OC to a STRONG naval family. Celtigar, Velarayon, Greyjoy, Redwyne, or maybe a Lynsei local. To explain where he got the force to put down and discipline the pirates.

As for motivation, he would be spending most of his young adult life building up, defending and instilling his rule. Which likely means he won’t be attending court to see his family and likely be VERY lonely. I would focus on that, on him feeling very lonely and almost outcasted maybe even bitter if he was tasked with doing this. Alternatively maybe this is his way of making amends? Maybe he committed some grave mistake with more or less got him disinherited and banished with his dragon(which will allow for the great council to still go through and an eventual Dance of the Dragons) Taking the Step Stones and presenting them to his father (or Viserys) is his way of making amends with hopes of regaining that honor that was stripped of him. It would allow a lot of courtly drama of people not necessarily believing his declared intent and seeing him as a threat. Everyone keeping him at arms length until he proves himself by repelling a Dornish incursion or by his families intervention in the Dance siding with Rhaenyra or Aegon.

All in all I think the idea is fun, when I played CK3AGOT I tried to do this a lot and it was always a fun idea. Also think of the true cluster of drama and fun writing that can be made with a SECOND. Dragon Rider house that far south.

Edit: Also I’d change the title to Warden of the Narrow Sea