At 70 AIP, the AI somehow got 3.5k threat. If they were smart enough to actually attack, they could wipe me.
The threat numbers are in terms of strength (ship count is meaningless when it includes fleet ships and guardians and whatnot).
Your starting fleet (flagship + triangle) is 1.6k strength. A single Mark 1 starship is 600 strength. A single cap of Mark 1 turrets is 1000 strength.
So 3.5k isn't especially concerning.
I have no idea were this threat comes from, I haven't moved my king unit once and have cleared every planet I have captured. It is frustrating.
When you show up with overwhelming force the guardians defending a planet will often abandon the planet and retreat to come after you later as threat. Is that a possibility here?
Also, I don't know if waves are sometimes being launched simply as threat, as we were doing that for a bit, but I think Badger made it not-on-by-default.
Turrets feel very under powered.
I'm fine buffing them again, but it would help to understand what exactly you're expecting. Currently turrets cost half as much science as fleet ships and have 5x the HP and DPS. That's a pretty stark difference.
When you have to spread them out so you maybe 5 of each type covering the many wormholes
How many wormholes are you covering? Is there another way to structure your defenses to reduce the number of "hard" entry points by allowing certain planets to fall while still blunting the attack?
I don't understand what you're facing that would cause you to need to do even that, however. For 1000 Science (1 low-science planet's worth) you can research 4 Mark 1 turret types. Added to your starting Mark 1 Needler turrets that's 5,000 strength. Yes, you have to spread it around to some extent, but that's a
really good deal on a strength-per-science basis.
Also bear in mind that you start with Mark 1 Tractor Arrays, which have a per-planet cap.
and AI threat moves in groups of a few hundred they really don't do much
A few hundred strength or a few hundred ships? If ships, 400 Mark 1 fighters is 200 strength, though for lower-cap stuff like missile corvettes it's 120 ships => 200 strength. Nonetheless, even that cheap 5,000 strength of turrets could stop quite a few groups of 500 ships each.
The AI able to destroy the turret controls at will because you cannot possibly defend them, there are too many and too spread out.
I did halve their number in 0.718. I could do so again, though it does beg the question of why-bother. The point of their existence is to have tactically-important terrain beyond "everything beelines from wormhole to wormhole or wormhole to controller". Metal/Fuel spots would be terrain, but they would not be tactically important most of the time. Is there a better approach we can take there?
Since my own fleet doesn't move faster on my planets I find myself chasing threat.
You can slow down enemy ships with a focused gravity generator (1000 science to get the mark 1). It has a planet-wide effect that's spread among all the enemy units on the planet (so the more enemy strength, the less the effect). It may need to be buffed again, as it received several nerfs due to the annoyance of the AI having it on so many planets (it's much rarer in AI hands now).