I look forward to your next one. My last was shat upon for having too many tanking mechanics (and, of course, requiring too many general changes, as my threads are wont to do). Would be nice to see something aiming for the comprehensive, but still more moderate/pragmatic. (I apologize if I'm off base in thinking that'd be your approach.)
I apologize if we've already talked about this before, but... this seems like really roundabout solution. I get that if tanks have or would have too much simultaneous output (enemy damage decreases + own damage dealt + indirect aid to party damage dealt beyond enemy damage decrease) for any single one of those outputs to feel impactful in its own right. But, why not just have their base mitigation be more reasonable, instead of starting them off with super-armor only to then have them apparently go out of their way to take more damage so that they only then return to normal "tank armor"? Realistically, one can make an active effort at the cost of dealing damage to protect oneself, but how would it make sense to have that much mitigation passively or for passive mitigation to be removed by performing otherwise normal actions?
Heck, you could have tank stance literally convert Attack Power into Guard (Block/Parry) strength, obviously with a revised parry system, and that'd make more sense. Heck, it could easily synergize into counter-attacks, too.
That said, I'm not sure why we'd necessarily need toggles, or even skill-swapping toggles, to diversify or give greater potential to outputs. We could just as well broaden GCD choices and their diversity of outputs alongside other job-unique effects and interplay. I'd sooner take minute imbalances than force a one-size-fits-all solution, and yes, I honestly think that diversity --if applied well-- only truly forces minute imbalances, rather than throwing balance out the window as some here would have us believe.