Hi,
a little thinks of doors have holes who we can look to the other side. But in the last newer versions have minecraft the texture fix what in 1.12.2 and little time between was give. I hope you can this fix. The flat texture was a so long bug in minecraft I hope you can fix it.

Picture:

Charming suggestion, isn‘t the perfect fit for a mod though as resource packs are already able to do so and change textures in the process as well. You‘d be best off with something like this: https://www.curseforge.com/minecraft/texture-packs/fancy-doors