Reducing tax liability with f/t work and one rental property

[Jeremy England]

Good morning all, so I recently did my taxes. I used Turbo tax deluxe. Anyway, it was a good year money wise. Better than usual obviously because we owe the IRS about 2800 dollars. I have 1 rental property and believe I have maxed out the write offs I can take without just flat out making them up. I had almost no expenses from the property this past year. I manage it myself, rehabbed it back in 2018. Advertising is free, software I use is free, computers I use are personal and prof use. I have an LLC specifically for the rental property.

Anyway, my net income from the property according to TT is -600, this is after depreciation and write offs.  

I decided to play with the numbers on TT to see if I could reduce my tax liability.  I wanted to see If I did any capital expenditures how much that would influence my bottom line.  The AC in my rental is pretty old.  Installed in 06, so I figured if I spent 3000 on that, what would it do.  My tax liability decreased by 500.  So I would "save" 500 on taxes if I spent 3000 on an AC.  

I've heard of retirement accounts for self employed people. I've maxed my IRA and 401k this year. But isn't there one where you can contribute like 50k or some ridiculous amount. Would that reduce my overall tax liability?

Thanks