I'm a huge anime fan and loved castlevania but I can see why it might not mesh with others. It's more like a good western show in anime style than anime. It very much ties characters with the plot and delves more into insights of how the characters react or are motivated rather than how the characters grow. They're all already the strongest in the world, so there's no traditional growth journey either.

If you want more like castlevania, you have to look for more seinen genre, meaning literally young man, as opposed to shounen, literally meaning young boy, which is what most popular action-adventure anime fall into (e.g. my hero academia, one piece, demon slayer, etc.). Parasyte is a seinen. Death Note is serious, but is ultimately a shounen and falls into leaning on its roots (le ebin outsmarting and powering up). If you want some recommendations, Jujutsu kaisen is really good and follows the Shonen frame of hero's journey, but doesn't lean on tropes and takes its maturity and realism in terms of characters a lot more seriously. Chainsaw man is also good so far. If you liked the castlevania show and want more gritty dark mature fantasy, try reading berserk. One of the goats. If you like fucked up shit try devilman crybaby. If you liked the philosophical aspect of Death Note, try Psycho Pass (stop at season 1). If you like seeing a colorful trio made up of 2 men and 1 woman go through various scenarios, some whacky and some depressing, try Cowboy Bebop and Samurai Champloo. If you're open to something a little more introspective and examines motivation and direction, try Ping Pong: The Animation and Tatami Galaxy (both I consider masterpieces).