Parry...

[ZDamned]

(And I'm not trying to pick on you - honestly I'm not! - but 1 is a soft cap, not 3. A soft cap is where a stat suddenly drops in value, but not to zero. 3 is a hard cap, where the stat literally doesn't do anything for you after a certain point. Diminishing returns are when a stat slowly and steadily loses value as it increases. REGARDLESS OF TERMINOLOGY, those are great questions and I hope to learn the answers too.)

No worries! I was under the impression that a Hard cap is the highest attainable from available gear, and a Soft Cap is the highest usable amount of a stat. I always thought Diminishing Returns were in a different category, but a lead in to what I had understood was the Soft Cap. So, thanks for correcting me!

Here is the thing: yesterday, i92 gear oriented parry, on T7, i was doing 125 to 130 in shield oath on the boss. Doing your best to not forget any SW/CoS when in dps gear, then slacking in tank gear is going to skew your results. If anything, the numbers you provided just proved that you should most definitely go in full parry because the dps you will gain by going with no parry and det and crit instead is going to be negligible (6 to 7% more likely, a bit more if you go with str accessories). Those numbers are on par with what you said and what i said, i.e foregoing all parry is going to make your dps in T7 go from 125 to 136.

Ok, Looks like there needs to be a consistent standard set on how much damage you can deal with, and without Determination/Crit, because that number you pulled makes no sense to me, unless your parser works differently than mine does.
If we ignore secondary stats all together:
I was using a Wave Buckler to maximize Shield Swipes. 3xSS are greater potency than 1 RoH combo, so I only use RoH when it's about to fall off, or when SB has connected, otherwise SS is higher priority.
Your lodestone shows you using the Noct Hoplon, If you were using that shield, you should generate way less opportunities to use Shield Swipe.
So that alone should give me a huge potency edge by the 3 minute mark in how much damage was dealt. (Unless you got super lucky, and blocked the same number of times as I did with a Buckler)
Which is the next question I want to ask you. How much total damage did you deal in that fight? I'll get my number and post it soon. (I'm at work now)
Also, did you Main Tank, or Off Tank? and Did you clear the boss?
On a Training Dummy, I get a clear 30~ DPS boost in my DPS gear over my Parry gear, and that is without Blocks adding more potency, so my in combat DPS is higher while standing still on a boss like T7.
So your stated number ALSO bothers me because I have more strength than your lodestone profile(If that is the gear you were in) by at least 10 points.
Your DPS number is either flat out wrong, or your parser is badly configured. Because stats can't enable you do pull those numbers.