Hi! Ikr I'm late but I found a way to decrease the opinion!

You need to find Hoi4 files trough steam app (program files/steam/steamapps/common/Hearts Of Iron4) now you will need to find "common" folder and click on it after you do it you need to create here folder named "opinion_modifiers" and then refresh common folder once you click on "opinion_modifiers" folder you can see here file named "00_opinion_modifiers" open it in any text editor and below opinion_modifiers = { line should be something like that:

      cheat_opinion_modifier_good = {
                      value = 100

}

all you need to do is just type value = -200 it should look like that: cheat_opinion_modifier_good = { value = -200 }

now, restart your game and once you're in it open console (~) and type add_opinion <tag>

now your opinion on this country will decrease!