Well, you are getting a bunch of drops including coins, so you'll eventually get a biome key. That said, I see a few issues:
16 water candles on one screen are no more helpful than one, likewise for heart lanterns. A Battle potion does stack with a water candle.
Eventually you'll need to go down there to collect the stuff, if only so your key doesn't get buried under stuff until it despawns 400 items later. Gonna need some chests....
Only a 3-second timer? You'd probably do better with a couple of 1-second timers or a Crab Engine, arranged so that alternate spears get triggered separately, by different circuits.
Personally, I prefer feeding most of an invasion to lava pits (tuned to burn only copper coins), over setting up traps, but I'm not trying to AFK them -- the only Hardmode invasions you can really AFK are Blood Moon and Pirates, and the pirates don't have that many enemies to kill unless you can trap the ship. I suppose you could stick your spawn point in your refuge, set up more spear traps to the sides (you know you can rotate them with hammers?) and just let the Mothrons and Reapers, or Goblin Mages and Summoners, kill you repeatedly.
When I wanted to get one of the keys before Plantera, I tried a bunch of things, what worked was taking a Solar Eclipse to the Hallow -- letting my lava pits kill most of the walkers, and fighting Mothrons and Reapers myself.