I thought Waves were always intended to be "Periodically the AI sends a kamikaze attack", which is a distinct behaviour from Threat ships (unless you specifically ask for the Waves to join Threat, like with Cross Planet Waves as implemented in AIWC).
So to my mind, anything spawned as a Wave should just kamikaze into defenses. I'd like the AI to have the choice between spawning those waves directly on the planet and spawning them at a nearby warp gate, then sending them to you. But either way, I think they should attack without delay, then press that attack home with some vigor (even if most of them die). One nice thing about this behaviour is sometimes the player overreacts and weakens their defenses elsewhere.
If you have a different conception of what Waves should be then that's fine (you are the Designer of the game, after all). But I still think my preferred behaviour should be saved as an option, since it's basically what AIWC did.
Edit: there's also a gameplay issue with the Retreat code in LongRangePlanning for the Threat Fleet. When it's choosing to retreat from a planet due to suddenly arrived human reinforcements, it seems to choose an AI planet at random to retreat to. In one example caused the Threat fleet to walk right past my arrived reinforcements and all die, instead of flying directly away from my reinforcements to a wormhole to a different AI planet. I think this code needs to be aware of where the forces on the planet are (or at least privilege the closest wormhole to retreat to, if figuring out where the human forces are in relation to the AI ships is too much effort for now). I opened this as
https://bugtracker.arcengames.com/view.php?id=19296 to make sure it's not forgotten; it's one of those little things that makes the AI look smart.
Also, the AI threat fleet has a nasty tendency to camp your wormholes, which makes it in many ways less threatening; you can always provoke a fight and try to whittle it down. I think having it lurk a planet or two away might make it much more scary to deal with.