Thank you for sharing!
I shared the same thoughts on the Avantone Planar, some songs feel quite close, some feel in distance. Based on your description of the K712, I think it might be similar to the Sundara? Wide soundstage but maybe too "wide" in some cases. Also, I do find some discomfort after using the Avantone for hours even though it is well distributed in weights. And that is why I put my eyes on the AKG coz they look more comfortable.
There is no doubt that the Avantone Planar is amazing in sound, but I want to find whether there is a can that shares similar quality sound but with a more consistent and rigid build quality. My pair has no QC problem so far, it feels "solid" but fragile at the same time, especially when you compare it with Beyers', I don't own dt1990, but I can feel its solidness in its design, instead of "fragile" metal build.
All in all, I really want to have a pair that has the same sound performance as the Avantone, but in a shape that lasts. Based on my shallow knowledge, maybe the DT1990pro is the closest one within the same price range that built by larger brand(under 500)? But I haven't really tried it. I don't know much about AKG lines........Sennheiser HD series isn't as neutral as the Avantone...