Recently had a bug that capped my deceleration and acceleration. I thought it was just a part of the game's difficulty until I realized I couldn't take off during a mission where you were under attack on the runway. Youtube tutorials are junk. Here's the real solution:
Right click the game in your steam library and click Properties.
Go into Controller and enable steam input.
Next, there is a underlined link above the override section called Controller Configurator . It will open controller settings for the game. Go into Edit Layout, then Triggers and this is the important part; change the behavior of Full Pull from Regular Press to Soft press. Do this for both triggers. Assuming you had the same problem that I had this will change it so that it will trigger maximum thrust and maximum deceleration. Its not a perfect fix because I no longer have the ability to half press and if anyone can figure out how to fix it better let me know. But I gladly give up the ability to half press for the ability to actually break and go fast and TAKE OFF.
I'm honestly not sure why this problem happened to begin with. Is it because I'm using a third party controller? Idk but I've never had this problem with any other game including Ace Combat 7 so I don't know why its doing this. Hopefully they patch it soon. Anyway, I hope this helped somebody! Please boost this if it did so more people can see it. Have fun and enjoy flying free!

EDIT: Its come to my attention that my controller was calibrated wrong! The calibration must have reset to default without me knowing at some point. Thank you Aurum Moon for the correction!

EDIT 2: So far nothing with calibration is helping so I might go back to soft press with steam input. What is going on with these controls?

EDIT 3: ULTIMATE SOLUTION: I finally figured out the problem or maybe the last problem with my controls. Full press and soft press are finally both available to me. IGNORE ALL PREVIOUS ADVICE. THIS is how you fix controller support for the game. After calibrating the controller on your computer, go into the Controller Configurator I mentioned before. Reset the Full Pull press settings if you changed it in the previous fix. Now, if your problem is the same as my the Soft Pull input will be greyed out for both triggers. The fix is easy, just bind the Soft Pull inputs to the respective triggers and you're good!

Just speculating here but I think the reason behind this problem is that the game does not check what kind of pull you're doing, it just assumes the first input it receives is the Soft Pull, so when you don't have Soft Pull bound it assumes Soft Pull begins where Full Pull begins instead, and your acceleration/deceleration never reaches the maximum because its treating the final half of your trigger pull like its the first half.

Its been a journey but I'm glad everything I'm finally done troubleshooting. Now I can finally enjoy the game how its meant to be played! Wish I found this out before making this post but whatever. Fly free everybody!