What are the final preparations for a Dungeon farm before killing Plantera?

I will say that Dungeon farming is tricky as I have built a couple different ones. The issue is that 2 of the Post-Plantera casters in the dungeon are painful and can bypass walls with their spells.

However, since you are doing this Pre-Plantera, A simple lava pit with a either a honey pool/campfire/heart lantern (or all 3!) on a floating platform can keep you healing well enough and eliminate the Blazing Wheel threat. Basic Dungeon Farming Pit this is not an AFK because of the chance of Ragged casters and a Diabolist now and again. (I have not searched out the tiled walls I need to remove, and the pygmies sometimes get bunched in the honey pit)

My pygmy setup, that unfortunately uses the staff so it is post-Plantera. Using a summoner group, like pygmies, ravens, or sharks (Duke Fishron staff) can keep the riff-raff cleared out including the Cursed Skulls and Dark Casters. Two water candles and a battle potion and holding a third water candle spawns thing very nicely if using summon groups. I like to put a Safe house (but I stole the statue for something else) out of harms reach with a king or queen statue in it to get a merchant NPC so I can sell stuff. I like to build it below the cavern level, where the Dungeon gets more convoluted.

The really nice loot comes after Plantera though, and that is trickier. I build an arena with just brick walls, and possibly some slab walls if I want sniper gear. The tiled wall creates the Diabloist whose attack is a persistent fireball that does lots of damage and passes through bricks, so I don't like that one. Ideally, the area you build should not have the tiled walls within a full screen from your safe spot to keep the Diabloist from spawning. Traps active, but I forgot my battle potions Notice the side walls around the honey pool. They are because the paladin is deadly if he has line of sight on you, and Bone Lee is just crazy fast and jumps very high, and finally, the sniper can shoot you from off screen if he has line of sight. So basically you need a spot that puts the spawns in harms way before they can get line of sight on you.

EDIT: Just for fun last night, I ran that farm for a little bit, maybe 15 minutes. For notable drops, I got 2 sniper rifles, a scope, a paladin shield and a bunch of miscellaneous but very saleable items. It filled a chest and half my inventory, for just sitting in the honey, and dodging the errant Ragged Caster that showed up about 3 times. So even without the Battle potion, that one works well for me. Since you can't see it well, in the pit I have a piggy bank and a safe, as it makes switching out stuff and storing coins easier.

What you should do is set up a honey/campfire/heartlamp/ healpad and get full tiki armor a raven staff, optic, staff, and a pygmy staff and then get a beetle thing ( i forgot its name but if you have a pygmy staff in your invetory the witch doctor will sell it) and a summoner emblem which increases minion damage and put your minions traps and lava to work. This works for afk. Even better is build a room down in the dungeon where you have your healing post and respawn there.