Live Show, LED Screens, Agh.

I've only done live sound once in a cafe so maybe someone more experienced can jump in.

That room is about 6500ft3 - pretty nice! Even so, it still qualifies as a 'small room' acoustically. Your best bet for sound is to bring insulation and put it in the corners behind the LED wall. You can also place some in front of the walls. I don't think this is what you're asking, though.

Spare compositions, in the low end especially, are generally a good idea. Like you acknowledged, the room is going to add decay to whatever sound you have. The current buzzword for EDM is 'shorten the kick'. Make of that what you will.

Since you're composing the music for this room specifically, you can account for the ringing frequencies.  8.5m, 7.3m & 3m will ring at about C#1, E1 & G2 respectively.  That means E Dorian will be a good key for the room.  G Lydian might also work.  Those two modes are from D major but I wouldn't use that due to the leading tone room fundamental and the G2 resonance instead of an F#.  A Mixolydian would give you emphasis to a rootless tonic seventh chord; though the E would help imply it.  I think A Mixolydian will be best, then E Dorian & G Lydian last. 

I mention this because I remember reading in Ethan's book that ringing can influence how we perceive the tonality of music.  In that light, you want to avoid keys which are one semitone away from the ones I'm interpreting the room to suggest.  Though dance music is often 'mixed in key' with perfect fifth, perfect fourth, or parallel modulations, the room here is a filter and time relativity (song-to-song impressions) won't apply to it.  For example, if you were to instead compose with D Mixolydian (a perfect fourth up), the C# room mode will clash with your key's minor seventh tone.  It would be fun if you try it out: compose a piece and transpose it from A Mixolydian to Ab or Bb Mixolydian and listen to whether you notice a difference.

From what I understand, a good starting place for a sub is 1/4 width along a short wall. Again, behind the LED panels should be fine. Alternatively, you can put it in front but then you have people there. You've got eight spots to pick from, including the ceiling.

The speakers will be tricky. I imagine that mounted high or above, angled down, in front of the led panels is best. You could also try just outside of them, angled in.  If you have to use floor stands then the latter option is probably best.