I suppose my question is: how large of a disparity in tankiness should there be between someone who has min iLV for the content vs someone fully geared up?
If it's too big it throws off how they balance content, it leads to healers being quite bored after tanks get too beefy.
I suspect this is why a lot of our tanking stats are de-emphasized.

Personally, I'd like to see them further de-emphasized and have tanking rely more on your skillz.
Buffing a stat doesn't make your gameplay more dynamic. If anything it makes your time playing less interesting as you can start ignoring more and more of a fight/dungeon/whatever.
Having a higher party stat doesn't change how I play vs having a low party stat. It's just an artificial gate asking if my stat is good enough to survive. It's like a min iLV check... but for a stat.
(This is all assuming they increase tenacity efficiency AND make it important to have, and not make it influence your build more, but you still dont need it)

Tying tankiness to ability usage on the other hand... expanding what tools you have and how you can use them will lead to a more involved and dynamic tanking, and it's a design angle that's obliterated much less by the player out gearing content.
And here the gate to clearing content isn't "is your number bigger than X? You are tanky enough to survive." But more about "can you use your kit well enough to survive these things and potentially improvise on the fly when things go skills.

Not that SQEX couldn't do both.
Not that there isn't a FF14 out there with big tank stats and more dynamic kits.
But then it leads back into players being able to be too tanky and the problems that creates.