The concept of the Stellaris combat cycle is good:

Design Ships -> Combat -> Review Report -> Use Results

Use Results: (Redesign or produce more of a class of ship)

The post battle report is lacking crucial information to making this possible.

In the post battle report we are told:

• How much damage is done by each weapon
• What the damage was done to (Shields, Armour, Hull)
• The accuracy of each weapon
  

For this report to contain more actionable information, I think it should include:

• Effectiveness of weapons relative to how many were used
• Effectiveness of ship classes
• Information on point defences and strike craft
  

A measure of the effectiveness of weapons relative to how many were used is crucial. If I used a hundred auto cannons then of course they did the most damage. The interesting information would be to find out if my two torpedo launchers did proportionally far more damage than expected. This can be extended to ship classes, which of my ships was the most efficient use of resources.

In fleets of 50 ships and 200 weapons we need to know what over performed so we can act on that. Not statistics that tell me what I already know before the battle.

Also, as far as I can see, point defence effectiveness and strike craft are unaccounted for in reports. Crucial to several fleet designs.

What other metrics would help us actually design and built better fleets?