Threatfleet is a collection of all AI ships that spawned for some other purpose, but lost that purpose and survived. For example, when AI ships retreat from a wave, or during a deepstrike/hacking action, they join the threatfleet. Another source is from Cross-Planet Attack Waves that don't immediately attack your planets. CPA waves can spawn (almost) anywhere.
The primary means of dealing with the Threatfleet is to go out and kill it. It won't attack you, unless it thinks it can win. For a large threatfleet, this may take multiple tries.
Without knowing a lot of what you have done so far, here's some general comments:
You have 18 planets. That's a lot - although you are beginning to approach the second AI Homeworld, so you can probably avoid taking more than one or two more.
-- Ask yourself - do you REALLY need all 18 of those planets? For example: Demurderxi and Moatbul, towards the bottom of the map, or Roadraswo and Linneiuh up top. These are isolated systems, and just abandoning them (letting the AI destroy them) may be a net gain, because of the reduced attack surface. Especially Roadraswo and Linneiuh - they are 4 hops away from your next nearest planet; not conductive to quick reaction support.
Your 18 planets are scattered. Only Ausare is not adjacent to an AI world. This leaves the AI a lot of vectors to attack you. This is related to your Threatfleet problem - the AI will maneuver to attack where you are weakest, and layouts like this give it a lot of choices.
-- Try to cluster your planets for mutual defense, and use chokepoints. When you can isolate AI systems (like the Bouzudon-Swojerwo-Buldasamu or Uberas-Sosuo-Rewyunburn-Asiur groups) kill everything in there (called 'neutering') and then you won't need to worry about Waves or the Threatfleet coming from those systems.
-- -- So, do some cleanup of AI ships in those systems. This will help you reduce the defenses needed in several of your systems, freeing up resources for use elsewhere.
-- Consider using Transports to move your fleet around quickly between your systems when they are attacked.
Your AIP is only 215. That's good for endgame, especially considering you have 18 planets.
-- If you can time your AI Homeworld attacks correctly, you can raise the AIP a lot during the process, but finish the game before it has time to bite you. Otherwise, though, try to avoid getting more unless it directly helps you end the game.
You have 23,500 Knowledge left. That's substantial, especially if you are having trouble. With 18 worlds, you should have acquired ~46,000 total Knowledge, so this represents almost half your growth in strength.
-- Consider upgrading you existing ships and turrets. Higher marks are much better than lower ones.
Counter-intuitively, Military Stations are the primary
defensive station, while Economic stations are the
offensive station type. Econ stations exist to produce more resources to help you rebuild you fleet faster. If you have Econ stations, you should be spending your ships! Let them die, as long as they accomplish something.
-- Suicide and hit-and-run attacks can help, if you have the right ship types. Similarly, if you can catch the Threatfleet when it is moving, you can defeat it in detail since AI ships move at their default speeds.
-- Consider Neinzul Starships as a support type. They can help significantly in attrition battles.
-- -- Try NOT to suicide starships. They are expensive enough that they should retreat when seriously threatened or damaged.
You seem to have a Champion active. What's the current status of your Champion? Is your energy currently being used by large allied Nebula fleets?
-- The Champion, especially if you've gotten a few hull upgrades, is immensely powerful, and a great way to clear threat without risk of resources.
-- Nebula ships are strong, but require lots of resources. If you have them, consider using them, even though they are expensive to replace.
You have significant hack points left. Hacking can be used to substitute for capturing systems (Fabricators, Controllers, Factories), to gain more Knowledge without capturing, or to debuff the AI. You may be saving these for the final AI assault, but consider using them earlier in the future - you'll get more, whether you want them or not.
-- If it is too late now, then just keep it in mind for future games.
If you haven't already, read
Kahuna's excellent guide to building defenses in your systems. His strategy can improve your defensive efficiency significantly, allowing you to survive otherwise fatal attacks or to redirect your defensive resources elsewhere.
If all else fails... use Warheads.
-- Yes, this will raise the AIP a little. But gaining a few points of AIP is preferable to dying.
-- Best usage would be to wipe the Threatfleet and Reserves with Warheads, then proceed directly into an AI Homework assault - that way, the AIP increase doesn't have time to matter. But, again - AIP is better than losing now (even though it helps lose later).
Finally. The primary* cause of death in AI War is the Threatfleet/Cross-Planet Attack waves. They're large groups of AI units, and they're smart. If you get to the point where it takes longer to kill the Threat then it does for the AI to replace it, you're on the edge of the death spiral. Not every game is winnable at that point. You can back up and try again, or start a new game. But continuing to play a doomed game is not everyone's cup of tea - consider abandoning this game if you can't figure out a way to win.
*The actual primary cause is probably carelessness ("Wait, when did that wave hit my Homeworld?"), but the primary *strategic* cause is Threat.