It seems like myself you also suffer from Terraria OCD ;D
However that does not mean you have to use box styles for everything.
For starters mix up some materials pearlstone goes nicely with stone slab because they don't "snap" together. I like using stone slabs as exterior material,giving shape and then define the insides with something else.
Also, double up! A 2 tile wall already makes a big difference plus you can experiment using a hammer to try different shapes. As a tip,google Fibonacci Terraria. That gives you a good idea of all sorts of shapes and the best thing is they are OCD friendly
I would get rid of the boss arena on top. Move it underground or in the sky if you still need it.
I would then convert the right side of your base into something like a cathedral with a nice big coloured window in the open section at the bottom. Or make a castle out of it
Google is your friend for references
Next, get rid of the background wall leading to the left to the 8 rooms stacked on top of each other. Give them shape and convert it to a watchtower that you could connect to the left structure via a hanging bridge.
Mix up your interior decorations, use different light sources and "theme" the rooms differently. Dungeon,glowing mushroom,corruption and so on.
I hope this wall of text isn't overwhelming and helps you get inspired. Keep on building!