What rewards should experienced players be able to get besides a design to plug into a shipyard that is worse than everything in their current fleet line up?
Well, that's part of why I suggested astronaut weapons and armor. Things that improve player effectiveness while exploring as an astronaut are necessary as well.
That being said, you touch on an important point. The biggest problem with...well, this entire game...isn't players like me, who dip into this game every six months and then walk away, though problems exist for players like me, or even the bare newcomer who knows nothing.
No, problems exist for players like
you, who are at the 'endgame'. Sure, you can build planet-crackers, or you can raid Pirate stations for fun (and sometimes profit), but in order to keep the veteran community playing, thought needs to be spared to the endgame.
As a 'filfy casual!', all of my suggestions are going to be biased towards newcomers and casuals. However, anyone who likes the game enough will eventually find themselves in your shoes.
It's possible my suggestion is a good 'partial' solution, but is just that - partial. As you note, it doesn't address endgame concerns. Sure, it gives a noob, or players like me a reason to explore. Why should
you explore, though?