Pretty cool to see this idea actually materialize! I wonder if you could use a shot of the dreadnought bridge in TLJ instead of using the shot of the Starkiller Base control room.

Regarding where it could go in the movie, I could share what I had in mind with this idea and see if it sparks any ideas for you.

I originally had this idea that when we first see Starkiller Base from space, we would also see a large First Order fleet in orbit. So dozens of tiny specs of distant Star Destroyers. Later, Hux gives his speech and then we see the fleet jump away into hyperspace. When Black Squadron is attacking Starkiller Base, we could hear an off-screen pilot wonder where their fleet is, having expected much more resistance. Then, during the climax we could cut to Hosnian Prime, where we see the First Order fleet jump out of hyperspace and engage the Republic Fleet. During the chaos of the battle, we see three dreadnoughts jump out of hyperspace and fire upon the Capital. I figured three dreadnoughts firing six rounds total would justify why the red blast seems to fill the entire sky as it approaches the senators on the ground.

With that in mind, I wonder if you could have three dreadnoughts appear in the sky above Hosnian Prime, rather than just one. Or even more ships to imply a big First Order fleet showed up. Maybe in the close up shot of the cannons firing, we could see other cannon blasts in the background flying down past the dreadnought.
I don’t think all of this is really necessary, but it might pack more punch if more than one ship was firing down on the city.

I do like how you managed to make a longer shot of the city with a dark night sky. If you did add a larger fleet to appear in the sky, it could be cool to see them firing at Republic ships that might also be in space. Imply that a battle begins and First Order will also destroy their “cherished fleet”.

Anyway, what you have so far is really cool to see!