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I know you said you don't have the manpower and resources to port the game to other platforms, but
because your using unity can't you just publish it to Mac/Web via the menu? It shouldn't really require any extra work (as far as i know).
Nice to see a fellow Dim 3 user!
If you would like some critiques:
The room seems big an empty, and colors are a bit bland.
The gun looks a little small? maybe center it slightly more, make it bigger.
The crosshair is default dim3- a cross would look better?