It looks alright for preview quality. The lower area is kinda dark, but not sure if that’s intended or not. The fake shadows on the crystal material look alright too, though it’s more noticeable when closer in the view to those. Perhaps adding some radial or other blur to the crystal material graph could make it a bit more visually appealing, or setting the material to subsurface profile. Depending on the type of crystal it is, whether a fantasy or a more actual type of crystal, they don’t have extensive shadowing with a ton of pronounced variation. It’s different when seeing crystal inside a cavern. But this is a cathedral where the lighting is quite a bit more ambient and even.
The marble walls and floor could look somewhat smoother / glossy. But I did read the backstory on the web page you posted, and it could be the marble wasn’t polished for reasons explained by the backstory / setting of the scene (don’t have the means to develop polished/glazed marble while flying around on giant pieces of land, and/or prefer not to due to their veneration of the helpful dragons). I’m offering prospective suggestions, not trying to critique on a deeper level.
I notice the pews don’t cast shadows now. Is it because the lighting was changed to be darker there? Is CSM (cascaded shadow maps) enabled in the directional light (termed light source in the World Outliner)?