First of all, congrats for trying your hand at FRED. It's a skill this community needs, but a hard one to sit down and master. FREDding a good mission can be overwhemingly complex at first. But you'll learn by experience and feedback. So you've made a good first step.
Now, about the mission itself:
-You need to give the player a specific goal during each phase of the mission. De-beam warships, dogfight to clear the airspace, shoot down bombers, etc. Ideally, your mission will be split into phases, each defined by a specific goal you have to complete. That way, the player knows exactly what they need to do, and get a new objective once they've finished the last one. Think retail mission structure; you always had directives telling you what to do at every point.
This mission has no goals, besides blow up destroyers (a task you don't have the bombs for, and therefore can't complete). Think of ways the player can affect the ongoing battle, and give them directives to match.
-Why is the player forced into flying a Ptah? If you want the player to fly a wimpy stealth fighter, give them goals that reward stealth (scan enemy ship X, get from point A to point B without being attacked by enemy fighters, etc.) For your first mission, you'd probably do best to avoid complex stealth gameplay, so give the player a more powerful fighter or bomber. Better yet, give the player a lot of ships to choose from in the Loadout screen, unless your mission is designed around the player using a specific ship.
-The Ravanas have their beams forward, and so won't keep them in arc if you just give them a basic Attack order (which makes them circle around the ship they're attacking). Pro-level missions give capships an elaborate choreography of waypoints to move through, to simulate the captains maneuvering their ships to bring weapons to bear. For right now, you might just want to swap the Ravanas out for Demons, which have broadside beams and won't turn out of their own firing arcs.
-Obvious Shivan warship name references are obvious.

TBH, you might want to try a smaller-scale mission before making huge epic battles like this. The more complex you make the mission, the harder it gets to keep it orderly and fun.
----
Hope I didn't crush your FREDding spirit.

Practice makes perfect!