since you are still working on this mod, (yay!) I have a few more suggestions!
first there are tons of weapons that are just terrible or a tier below what they should be by the time you get them, eg. wand of sparking, Life Drain, crystal vile shards, venom staff, Venus magnum, the earth boulder staff etc
The friendly NPC really do need a buff in expert mode/ hardmode. No matter how protected your town is, they always ended up dying at least once in hardmode events with wall-phasing enemies. they do respawn after a while, but the party girl takes an expectionally long time to respawn. Also, it is very disheartening that a single deathly sphere in expert mode can slaughter an entire town of NPCs by itself. I don't expect them to be legendary fighters, but at least an individual npc need to go toe to toe with a wraith or almost down a deathly sphere to 30% hp in expert mode. They need to double the health in expert' mode (like their hostile counterparts), and they need an overhaul that gives them weapons with better damage and fire rate in both pre hardmode and hardmode.
in terms of the armours I would say all the original hard-mode ore armor need buffs, since they are vastly inferior to their counter part in terms of passive abilities and armour ratings. the chlorophite armour could also uses a buff to be a more viable option.
Bosses wise, the Golem is too easy to farm, It is meant to be harder than plantera except it is not. Maybe alter the drop rate for the rarer items as well if the boss is going to be tougher to farm. the expert item, brains of Cthulhu, is underwhelming compared to the wormscarf, which is ironic considering the brain of Cthulhu is the harder boss.
hopefully these are the suggestions you are looking for!