I have my trim set to two buttons, up and down (I have a basic controller so this is the best I can do). However when using basically any aircraft besides A2A's Cessna, and especially Captain Sim, just one quick click sets the trim way to much. Is there a value any files that I can change to a lower/slower value so one quick click doesn't affect the trim so much? Or is there any other alternative?

Checking in your Settings/Control Buttons, the 2 controller buttons that are programmed for pitch up/down, do these have

 

sliders in the next column and are the sliders to the right?  If so, move the sliders to the full left.

Yes, I had the same problem with CaptSim planes where the trim sensitivity was too great. Just go into the aircraft cfg and look for the trim entry and change the scalar. It might have a value of something like 1.0000, you can change the value to sometime like 0.6000 and test. If its still too sensitive lower it to 0.5000 or lower. Conversely, if 0.6 is not sensitive enough go to a higher value like 0.7000.

If you have a registered FSUIPC, there's also a checkbox to fix the trim rate.

Or try the Saitek Cessna trim control wheel.  Very nice addition to your sim hardware stable.  You will need the Saitek driver for it & you will need to disable the trim buttons on your yoke in the buttons & controllers page.