You should continue the crimson king, it's such a deeper premise than with canon. Rhaenyra's brother(half) aegon was born and he wasn't heir, he wasn't raised or brought up thinking he was the heir, so rhaenyra being heir wasn't a bad thing for him because he had nothing "taken" from him. But this guy your OC was raised thinking he was the heir, he grew up thinking he was going to be king. Only for his father to make his older sister heir. That, that is a story