Advice for Next Purchase

Hi all, I'm looking for some advice for my next brewery purchase. I have about $100-$200 burning a hole in my pocket and I want to get the best bang for my buck. I'm currently an all-grain brewer brewing in a 15 gallon mega pot with a cooler MLT. My girlfriend gave me a March Pump for Christmas so my thoughts are taking my money and building a "Jamil Chiller". I also recently completed a STC-1000 build and know I should bite the bullet and buy another chest freezer. I'm also thinking of spending that money on various fittings to put a ball valve, sight glass and thermometer in my kettle. I want to get some kegs and turn those into a HTL and MLT to eventually move to a single-tier setup.

Here is my current inventory:
15 gallon pot
Two 10 gallon coolers for HLT and MLT
Banjo Burner
Stir Plate
Keezer with 4 taps- 6/7 kegs
Yeast washing/starter equipment
Monster Mill

What should my next purchase be for the best bang for the buck and to ensure I won't have buyer's remorse?

Unless you just missed mentioning it, I'd do something for fermentation control. Get a temp controller and a small fridge or freezer (used).

Plumbing and hoses for the pump cost about the same as the pump. So unless you have these I would go with that.

Just realized I neglected the STC-1000 mention, so ferm temps are taken care of. You definitely could go for some disconnects. I just did. But man is it depressing dropping $100-200 on a couple handfuls of little bits like that. More fun getting something more substantial for your money.

Get an aeration kit. It will do so much for your beer.