How can you change graphics options (anti-aliasing, texture quality, etc.) of a 3D game with GDScript?
(It must work in the exported game, not only in the Godot editor.)
Most graphics settings can be changed at run-time by calling methods in VisualServer or modifying properties in the Environment resource currently in use. See this script for an example.
Godot supports shrinking all textures by a 2× factor, but keep in mind this also affects 2D elements. Note that texture quality generally doesn't impact FPS noticeably, so I would leave it as-is at first.