Something that often falls through the cracks in the story is that the Ul'dahns do have a complicated relationship with the thaumaturges. You'd think it'd be because their practices are adjacent to black magic, but the connections were cleverly hidden in funeral rites rooted in worship of Nald'thal, so that rarely comes up.

Rather, people look at the thaumaturges with suspicion because they spend a lot of time around death and embrace sin as part of the vital human experience. Even when they are honorable in business dealings, funeral arrangements, crafting legislation, etc., they still work with "umbral" energies as much as they work with "astral" ones and preach the value of sin. The thaumaturges, however, merely don't care. They regard such people as having a myopic worldview.

To the thaumaturges, to ignore sin is to deny the context of real truth. To outright deny it when it ought be denied and let it weigh on your soul when it ought to, this is living righteously. Such knowledge of the truth lets you see the world of Nald more clearly, and in fully understanding the world of Nald, you can look beyond the veil into the deeper truths of the world of Thal.

Part of a thaumaturge's training (at least in the past, I'm not sure if they still teach it) is to meditate within Thal's Respite until one can hear the prayers offered to Him. While one may not divulge what they have heard or from whom, it serves as a lesson: the "good" people of Ul'dah who look down on their "dark" ways, when no one as looking, still grovel before the god of death and ask Him to curse their enemies with misfortune and illness, to destroy love they don't approve of, and so on. They hide and deny their sin, but they sin all the same - if not worse, and deceptively.

Their judgement means nothing.