Most important next purchase to make my brew better

I’m just looking for some opinions on what would help me make better beer. I currently have the following:

5 gallon stock pot to heat strike water and sparge water for batch sparging.
10 gallon rubbermaid cooler from Home Depot as a mash tun
10 gallon SS BrewTech kettle (on it’s way to replace the 8 gallon aluminum pot I have been using)
Bayou Classic burner
copper immersion wort chiller
2 6 gallon glass carboys
2 5 gallon glass carboys for secondary fermentation when I choose to use them.

What is the next thing I should consider purchasing to make my beer better? I don’t have any way to control my fermentation temperature other than keeping the carboy in an upstairs closet or my basement depending on the range of the yeast I’m using. Should I look into a mini fridge/freezer with a temperature control unit that will control my fermentation or is there something else I should consider as my next step?

I would lean toward fermentation controls, but don’t discount some stuff to do yeast starters. That would be a couple of flasks, DME, etc. You don’t need a stir plate, shaking is adequate, but might be something to explore.

Sorry, I forgot to mention I do have a flask and stir plate was well for doing a test started when needed.

I once was at the same level of equipment as you, and the next things I bought (in order) were a used 10 cu ft refrigerator with a Johnson controller, BeerSmith software, a yeast starter kit, and Bru’n Water software.  John Palmer has a good video on all-grain brewing, with 5 things to help make a better beer - proper sanitation, controlled fermentation, use of yeast starters, mixture of malts, and water treatment - I watched it to help me choose my next brewing equipment first.  Look for it on YouTube.

Temp control is a no brainer

Yes!
And make sure the controller you get does both cold and heat at the same time.

Temp control above everything.

Is there a good recommendation for a controller?

Temp control is always my first recommendation for somebody at your level.  Best ROI for making better beer.

Chest freezer plus temp control is a top option. The other is your own grain mill if you don’t have one. I went with the mill first, since I have a basement that works well enough for fermentation temp control for the majority of my beers. Having your own mill leads to consistency in mash efficiency, and allows you to purchase unmilled grain in bulk.

For a dual stage controller, I really like this one:

I agree with the others on temperature control.

I would add something else. Brewing Better Beer by Gordon Strong. Small investment with loads of information to take your beer to the next level. I’m still working on changes suggested in the book.

I have one that’s easier on your wallet: It looks like you do primary in carboys. A good old fashioned fermenting bucket is easier in every way.
I agree that temp control is key, but if you live in the north, have a cellar, and are willing to brew somewhat seasonally, it doesn’t need to be an expense.

+1, been very happy with mine

If you’re getting a bit long in the tooth like I am an upright freezer might be easier on the old back. When my chest freezer crapped out a while ago I went with an upright.
#1 on the mill.

If you live somewhere cold enough, an aquarium heater, ice packs and a water bath can do the job.  That way you can save money for kegging.  Of course, there is always a need for more gadgets.  Since I already do water baths and kegs, I’m waiting for a fridge or freezer to come my way for even better fermentation control.

This^^^!

First. Before a big brew kettle even kegging. First upgrade. Unless one has a magical spot in the basement/house as some have reported.

I’ve seen folks spend nearly $300 on a cool huge boil kettle with a valve and a sight glass and a thermometer,  but pass on doing temp control until they can afford it. Makes no sense to me. Reminds me of kids back in my high school days who put a huge decked out stereo in their 74 Pinto with bald tires. $300 invested in a small freezer and controller will improve beer infinity more than a $300 BK

+1000

I guess that all settles it.

I’ve had pretty good success with keeping good temperatures for the various yeasts in various places in my house but I would love to be able to move to the next step of lagering at some point as well. Looks like the search is on for a good size chest freezer to use. I’ve seen some in the range of 7 cu ft that look like they are big enough to hold 2 carboys in the event I have 2 batches going at the same time so that may be the route I go unless anyone has any suggestions based on what size the use.