Reduce GPU Memory usage from textures

i have a huge scene with a lot of high detailed textures. As a result the scene currently needs about 2GB GPU Memory. Since i can't expect everyone to have so much GPU Memory, is there a way to reduce the memory usage?

A few ideas i have already tried:

• Only a small part of the scene is visible at any time. Splitting it in smaller scenes should therefore be a possibility, but when loading the additional scenes in the background, i get framedrops or the game is frozen for a few moments. Besides that, i can't be sure a part of the scene is loaded in time.

• Using AssetBundles had similar issues, besides additional code.

• I have nearly no RAM usage, is there a way to move unused textures temporarly to it?

• The textures are compressed and mipmaps are turned off.

• Different Quality-Settings didn't change anything notable.

Sidenote: CPU, GPU and RAM usage are no issue.

edit:

A bit background information, in case this helps: I'm creating a 2D-RPG with HD graphics and big levels. Think of it a bit like older Final Fantasy or Pokemon Games, but i'm not using tiles, instead a more natural/artwork-like style. Therefore the textures are bigger and require more space. The level-size is usually somewhere between 20000x20000 and 60000x60000px, on multiple layers and split on multiple parts (to bypass the 2048x2048px texture limit on some devices). Huge sizes of assets-files are expected, but i'm searching for a way to reduce the size in the GPU memory.