The main way in which tanks differentiate themselves currently is positioning. I know that many tanks go through a phase of being convinced about the overwhelming significance of their DPS contributions, but nobody else shares that perception. This is just common sense - less of your toolkit is dedicated to doing damage because of mitigation bloat (taking up more than 1/3 of your actions) which in turn makes your rotations intrinsically simpler. Your contributions are evaluated and weighted by the devs at significantly less (in part because they want you to be able to fully concentrate your mental energy on invuln-swapping the 4-5 fixed-timestamp tankbusters without dying and accidentally causing a raid damage loss through a swiftcast-raise). In return, you can feel super important in dungeon content and not have to worry so much about vuln stacks on failed mechanics. You can be the big fish in a small pond. That's likely the main reason why some people still defend the progressive gameplay simplifications to the role instead of speaking out against it. Which is a shame, really, because you should want to encourage competitive-minded players and give them a skill differential to distinguish themselves for good performances.

Mitigation checks are pass-fail. A good mitigation check will make you ration out your cooldowns, and historically you wouldn't have needed to draw on end phases of ultimates for outlier examples of that. That's not entirely expected, however, because most players aren't interested in sitting around on their hands watching a support prog their mitigation strategy. You're largely interchangeable with a lot of other tanks equally capable of doing the exact same check at the same level of consistency (especially if you're on a half-decent DC). I think invulns are a big part of the problem, since they generally let you remove a tankbuster of your choice from the equation and let you bypass swap/split mechanics. Holmgang's recast in particular is much too low, even after being raised to 4 minutes. At minimum, the recasts should all be longer (probably in the range of 6 minutes). Alternatively, they should find a way to phase out invulns with a tier of high level mitigation (i.e. something in the 80% range) so that you're still forced to do mechanics and ration out cooldowns, although that may require revising some damage numbers on older content. Another option would be to routinely incorporate debuff effects that make incoming damage just flat out ignore invulns and force swaps.

I think the current tank approach to self-healing also detracts from your mitigation experience because all the emphasis gets placed on all or nothing tankbuster-level damage. Many of the pre-Creator fights had a lot of outgoing damage in autos and cleaves that would flat out just kill you if you didn't work as a team with your healer (or perfectly time short mitigations on unmarked cleaves, like on A3S). Part of the tanking experience involved sitting at extremely low health totals and knowing whether the next auto would kill you or not if it crit (as well as knowing the precise timing of said auto). These built-in rotational regens and on-demand burst heals are just there to comfort new tanks (especially the type of player who tends to chant at their healers to 'HEAL' when they drop below 90% HP) and make tank gameplay more or less stress free.

No, positioning checks are the last bastion for tanks, and Criterion does have some decent examples of this (especially Rokkon). I get the feeling that the fight designers mostly gave up on manual positioning after Midas, but you do see some moments of brilliance every so often. This is a much better soft check because you're trying to maintain uptime for both yourself and your DPS while anticipating mechanics. Incorporating more random elements (bees/cats/bouncing metal balls) represent another way to keep this fresh and interesting and introducing some moment-by-moment tank decision-making, but my feeling is that they will always hold back massively on difficulty when it comes to random elements like that (even ones that can be baited).