Hi Sam!
Good song - yes, too short, but leave them wanting more, as they say. I like the attitude in your vocals - perfect for the theme.

Mix-wise, all good. I would have preferred a bit more volume. In Reaper, I normally aim for LUFS-I of -14. If this is new to you (forgive me if you already know - I leaned about it on these forums) you can see this in the Render window after doing a dry run. If you’re getting clipping when doing this, try adding the Cockos ReaLimit to the master track, adjust Brickwall ceiling to -0.1 and play with threshold until you avoid clipping and increase the LUFS.
Andrew