So if I understand your setup and situation correctly, your monitor and GPU all work fine when running at native resolution, the 4K setting. But when changing to a lower resolution there is something wrong.
The first thing is to be sure the display is setup correctly in Adrenalin. The Display tab should have GPU Scaling set to On. The Scaling Mode should be Preserve aspect ration. I also have Virtual Super Resolution as On.
Next, in Windows Settings, in Display, the Scale and layout section have settings for the display resolution and the size of text, apps, etc. In Display resolution you should be able to select the lower resolution you want. You can also change the scale. My old eyes prefer 125%, but at a lower resolution I can probably get by with 100%. The combination of settings here should get you where you want to be.