I'd say his aspirations are pretty mundane. Security and stability are probably the biggest priorities overall, financially and health-wise. He doesn't thrive in unreliable and unpredictable surroundings. The fact that he knows he will have his basic needs met for the foreseeable future, there are people ready to prepare him a warm bath at a moment's notice, a reputable doctor to look after him, and armed guards that are never too far away, eases his mind considerably. A large part of his work revolves around routine, carefully crafted plans and immutable etiquette, with relatively few unpleasant surprises. He's so high in the hierarchy that very few people can treat him disrespectfully and get away with it.
He wants to prove that he's capable, competent and useful. His deeply rooted inferiority complex (that largely stems from the demeaning and belittling way his mentor treated him when he was his apprentice) has made him a lifelong overachiever, which in turn has served him well in his career. He's ambitious and driven but I wouldn't call him power-hungry in an egoistical way, he can come across as overbearing but it's because he's a perfectionist control freak who's obsessed with doing his job well and has a tendency to think most people around him aren't up to the task. He isn't in it for fame and wealth in itself, it's more about having a purpose that makes you worthy of respect.
On a more personal level he's passionate about reading, studying and learning. Partly because he's inquisitive and genuinely enjoys it, knows he's good at it and feels good about being good at it, but also because he wants to be the most learned, most cultured and most academic person in the room. Not necessarily for bragging rights, but to feel like being smart will always keep him one step ahead of the others and that way no one can pull the rug from under his feet.
He would never be able to afford the things he wears and the luxuries he has access to if his life hadn't taken the exact turns it did. He spent his early childhood in a monastery and was trained by a priest who valued asceticism and self-denial so he didn't have a lot of nice things growing up. Now as a high ranking church official he has more spending money than he could've imagined, and while he has an expensive taste, he oftentimes fails to enjoy the benefits of his status properly. He has a comfortable home with a massive bed, but it's not uncommon for him to sleep in his office or forgo rest completely. Even though he could be savoring the rarest most complex dishes every day, there aren't a lot of foods he likes eating. He would like to look pretty but even his outlandishly costly and carefully tailored silk garments can't redeem the fact he doesn't feel comfortable in his skin.
He can't resign because his sense of self-worth and lifestyle are tied to his job. It's the one thing he's demonstrably skilled at. He's worked himself to the bone to get where he is now and the prospect of losing it is simply unfathomable. He doesn't have ties to his biological family and his friends are few and far between, if he gave up his position he'd have functionally no one to rely on but Vasco. On top of that he does feel like he owes his life to the church and serving it to his best ability is his lot in life. His state of faith and relationship with God is complicated at best but he's nonetheless terrified of what might happen and how he might be punished if he ever chose to abandon his post.
