even with a new property to allow certain weapons to ignore it (which seems much, by the way), i think making the damage equal to CON score is too much. you're looking at 15-30 damage for a significant plurality of creatures on a single attack just to make them roll the save.
i think maybe half con score would work, though. or maybe con mod plus proficiency bonus (i'd've said con save bonus but there are monsters without con save proficiency and negative con, so...). the former maxes out at 15 damage, and the latter at 19.
making one maneuver a prereq to another is just...flat out not currently a thing to my knowledge, even with maneuvers where it'd make complete sense (e.g. you don't need whirlwind strike to get whirlpool strike). other then that, this isn't a bad idea.
As I said, I don't know the maneuvers well. I'm just throwing ideas out there.
Fair on the weapon property. I would expect it to be rare and size limited but might be too much to do in this system, so I can drop it.
As for greater than Con score damage to stun, in my group, the bard does a battle hymn that lets them add an extra die. Then they have a maneuver to do that or maybe I gave them something I shouldn't have.

In any case, the fighters do 15 points average, so wouldn't have a problem stunning most creatures. I see the point that Con score is too high. What if the target doesn't have to save until they take more damage in a round than their Con score? Then it's not that one hit stuns but a series of blows meant to stun? This also stops the fighter from running around the battle, stunning enemies. Or what if the maneuver required all hits to be against one target and the save isn't until the attacks are done?
Thanks on the other idea! I'm surprised they don't have prereqs. Maybe it's assumed that if they are taking stunning assault, they have prior maneuvers in it? If not, I wonder if they should? Otherwise, don't we have a maneuver dip problem? That is to say, the fighter gets access to a new second degree maneuver at fifth level. They can just take Stunning Assault having taking no other maneuvers in Tempered Iron? Of course, as I read the earlier ones, none are like Stunning Assault. I might even argue that particular maneuver doesn't seem to fit the rest of them. Again, just talking "out loud" about this as I read them.
"Characters get Stunning Assault in tier 2- that's also when magic items become more prevalent, like potions of Flight and winged boots, and spells like Fly.
Jump and Web are tier 1 spells that can help with this as well. Anything that prevents a flying creature from moving means it drops to the ground."
I mean, that's assuming the casters have those spells or the enemies have those items. While it would be good tactics to get off the ground if they do have those things, it seems a bit metagaming that bad guys keep showing up with those things. I'm not saying you said that. I'm saying I would like to find other ways to deal with it as stunning an enemy is huge.
Is there a skill or way to tell what combat traditions a person has? Maybe by how they stand or hold a weapon? Or they don't know until the fighter stuns someone? Some way to defend against it? What if the Shield spell or a shield could be sacrificed if the save is failed to negate it?
Wow. There is no spell I can find that cures the stun condition. Nothing in the Restoration spells or even Heal. I think this makes Stunning Assault more OP for its rank. Maybe the condition itself needs to be reworked? Or the progression of Rattled -> Stunned needs to be used? The fighter can potentially shut down opponents they are near equal to their number of attacks, assuming they fail saves.
I'm also wondering if it needs to be limited to humanoids? I had suggested by size before, or larger creatures getting a bonus, but now it seems strange to me that a dragon, fiend, or aberration can be stunned by this. Having said that, I could get behind a character who has a studied target being able to stun those types.
The other reason I'm thinking about this is that the movies don't have the BBEG's with minions. The dragon should be enough of a threat on its own. The game mechanics don't support that. All bad guys should be in groups when attacking. The reason I don't like that is this mostly extends the fight rather than making it cinematic.
Again, just thinking out loud.
Thanks for the conversation!