I would take some water to the LFS to have the ammonia checked, and to determine if your ammonia test kit is defective. You have plenty of bio media for 2-3x that stock.
After you rule that out, ensure your mechanical filtration is clean and is placed before the flow of the bio media. If not, your bio media may be getting clogged and less able to perform. I'm sure you know not to clean all of the bio media at once, or you'll have a huge bio bloom.
Lastly, feed less. Try skipping two or three days a week.
FYI, I have the same size tank with at least 3x that stock and I haven't cleaned my 4L of Matrix (~100GPH) in over a year. I use about 6L of pillow stuffing for mechanical with a ~300GPH flow and I change it about once every month or two.
Edit: I typed all of that up after you showed the pics of your tank, and before seeing your water test. What everyone else said is right, your ammo is fine. Try restricting the flow in one of your filters with the most media. This may help reduce Nitrate.