I'm not sure what you mean by "super grainy", but Eu4 doesn't have proper UI scaling and the text is often described as blurry, especially when using fractional scales. Do I understand you correctly that the notch isn't a problem on fullscreen with 3024 x 1890? Then you could try the mod
Proper 2K UI Enhanced (with an UI scale of 1.0) which scales the whole UI by about 1.3 and includes many other UI improvements including fonts which are easier to read.
Or you could try to get a fullscreen resolution of 1512x 890 in some way. Did you check both the launcher and the ingame video settings to see if one of them might have it? If not, you could also try to set it manually in pdx_settings.txt and settings.txt. IIRC the launcher automatically adjusts the resolution setting if you open the game settings tab while it is set to a resolution which isn't in the list, but I think you can avoid this by not going into the game settings in the launcher