Well, the experimental results:
Only the Trader can save you.
This time, as part of getting more stuff to be strong, I ended up going all the way to Battleships accidentally. So, the response was about 236 Nemesis frigates from each AI.
My homeworld was about as optimally set up for this sort of defense as I could figure out, with one entry wormhole the Exowave could come through, and with a distance of about 65,000 between that wormhole and my Home Command Station.
The map also conspired to help, as there was a Grav Drill between one AI homeworld and my Homeworld, so one of the two batches arrived more than 2 minutes after the other.
I played the game normally for the first 4.5 hours, with no cheating, just planning ahead.
I built 4 Mod Forts, all 5 Mk I Fortresses, a huge selecting of mark I-III Grav Turrets, mines, Mk I-III turrets, Mk V Sniper and Spider, all supported by a Mil III Command Station, of course.
With Trader goodies:
With Armor Inhibitor, Radar II, 2 OMDs - Survive.
Without Armor Inhibitor - Die.
Then I did some resource cheating and built the SuperFortress, too.
With SuperFortress and Radar II - Survive.
With SuperFortress, 2 OMDs - Die.
With Armor Inhibitor and SuperFortress - Survive.
With Armor Inhibitor, SuperFortress, and Radar II - 'Easy' win.
Then I did some more resource cheating, and built all 25 Mod Forts plus all the Mk I-II normal Fortresses.
With no goodies - Die to the first group of 236.
With only 2 OMDs - Die.
With 2 OMDs and Radar II - Survive.
With 2 OMDs and Armor Inhibitor - Survive.
Then I did some more cheating, and made it so the second group arrive within 30 seconds of the first. This went less well for Humanity.
With all the Fortresses and stuff, no Trader Goodies - Die.
With all Fortresses, etc, and Armor Inhibitor - Survive.
With all Fortresses, etc, and SuperFortress - barely survive. My Home Command Station took damage every time I ran this test.
With just the original defenses (Mk I forts) and Armor Inhibitor, SuperFortress, Radar II, and 2 OMDs - Lost 50% of the runs. Home Command Station FF network took lots of damage every time.
With just the original defenses (Mk I forts) and missing even one of (Armor Inhibitor, SuperFortress, or Radar II) - Die.
So then I did some additional cheating, killing off some of the Nemesis as the were inbound, to control group size.
Using No Trader Goodies:
Two groups of 100 arriving simultaneously - Die.
Two groups of 100 arriving 2 minutes apart - Survive, barely.
Two groups of 150 arriving 2 minutes apart - Die.
Basically, with strong homeworld defenses and a HUGE distance (75% of the entire system) you have enough time to kill about 150 Nemesis Frigates before they kill your Home Command Station. Trader goodies can increase this: Radar II makes the Nemesis get a little closer to each Grav Turret before being able to kill them, keeping them slowed longer. The Armor Inhibitor basically does 20,000,000 damage to each Nemesis when it enters the system. The OMDs kill one very 10 seconds, and in this case accounted for 10-ish kills each.
At short range, where the entrance wormhole was only 30,000 distance away, a group of 100 Nemesis is able to destroy the Home Command Station before you can kill them. Again, Trader Goodies can help, Armor Inhibitor in particular, but it's a race of their firepower against your Home FF network, not your overall defense strength.
I would suggest that response intensity 10 be aimed at producing a total of 150-200 Nemesis Frigates for 8 Battleship Champions. That's enough that it would be very hard to survive without Trader Goodies. A multi-system defense in depth is required at that point.
Intensity 5 with 8 BB Champions should be a total of no more than 100.
Intensity 5 with 1 BB Champion should total around 50. In that case, you've already unlocked the stuff that's actually going to help defend yourself - the Mod Forts and Nebula Ships outweigh the Champion itself.
So ultimately this does look largely provoked:
Yeah, it is pretty much self-inflicted under the current settings. I just don't think it should be instant-death at time X. Even Astro Trains Intensity 10 isn't that sharp a cliff.
That said, I think I'm just going to remove the highestChampMark factor entirely, as it does feel a bit treadmill-y and clearly the math is just fine without it.
Actually, my first instinct would be to do the opposite - Up the Hull Size factor, and lower the per-Champion factor. However, your numbers would result in, hmm...
26,000 * 2.4 (count) * 1 (no size factor) * 4.5 (Diff 9) * 1.5 (Intensity 5) = 421,200; 421,200/2480 = 170 total Nemesis frigates in Exowave 1. 212 in wave 2.
26,000 * 2.4 (count) * 4.5 (Diff 9) * 6 (Intensity 10) = 1,684,800 = 680 total Nemesis.
26,000 * 1 (count) * 4.5 (Diff 9) * 6 (Intensity 10) = 702,000 = 284 total Nemesis.
26,000 * 1 (count) * 1 (Diff 7) * 6 (Intensity 10) = 64 total Nemesis.
26,000 * 1 (count) * 1 (Diff 7) * 1.5 (Intensity 5) = 16 total Nemesis.
Edit: Attached the image of my HW defenses: