Making that material
Should be fine. Looks like theres no displacement, no normal map and no translucency, so it should be fine as is.
Always use 1 material slot?
Nah use as many as you need. Objects can be assigned more than 1 material (by way of multiple material slots) so you have more control over how the object is displayed. For example, if you had a house mesh with windows, youll want a different material slot for the house and a different material slot for the windows. Check out the Paragon characters on the Epic Store and youll see how Epic used material slots.
Translucent UI
In VR? Like iron man’s HUD or something? I mean, its not going to be as performant as non-translucent UI, but should work. Most of these things youre asking are ‘use your best judgement’ scenarios that are going to depend on the demands of your project. As a rule, if theres something you want to do, just do it and see how it performs and if its not good, then think up ways to optimize.
Preferred build settings for Quest 2
I dont usually work for standalone Quest projects, but you should be fine with default settings as long as you have sensible lightmap resolutions set in your static objects. Higher resolution = more RAM required. There are profiling tools for this in the engine that should help 