Sorry for double posting
Ok: Here is the very ugly and dirty workaround to reduce the heavy RAM-usage until there is a proper solution.
1. Go to your steamapps\common\Galactic Civilizations III\Gfx\Models\Ships\Textures
2. Sort files by size and delete all files above 5 mb
Notice: You should make a backup of the files first, but when moving them into a sub-folder the game still seems to find them. I made a quick .zip instead.
This is a first-aid-treatment, no solution!
The Treatment reduces Game-Data of ~2.2 Gb. This will of course have a visual effect on your game, though there is no error-message or something. Also there might be issues, that i haven't experienced yet.
The Ship-Models and colors still seem to be okay and it's playable, though they might flicker a bit due to missing textures/shaders/maps.
The problem: The game seems to use multiple and really big texture-files per shipset.
e.g.
AltarianTexture_A_Data.dds 42.6 MB
AltarianTexture_A_Lean1.dds 42.6 MB
AltarianTexture_A_Lean2.dds 85.3 MB
AltarianTexture_A_Normal.dds 42.6 MB
AltarianTexture_A_Diffuse.dds 10 MB
AltarianTexture_A_Light.dds 10 MB
Total: ~232 MB that has to be crawled over and over again every time it draws an altarian ship. Not sure if the visual benefit at a common zoom-level justifies the texture sizes in general, but for me it doesn't ofc. So, for the 4 GB ppl (and 8 with background programs running), the game seems to need an optimized texture pack or the option to disable/reduce certain texture-effects.
There is definetly more room for optimization or a more elegant workaround, but i dont have time for further testing, i have to stop a Drengin Invasion!