Paladin's get Sheltron at a low, somewhat reasonable level of 35, but the rest don't get theirs until 56, 68 and 70, for Warrior, Gunbreaker and Dark Knight, respectively.

But, with tank buster markers being introduced in all older content, and some of these coming faster than Rampart can come back up, I feel like all the tanks should get some form of quick mitigation as early as level 15.

Paladins still get Shelltron, but make it like the old version of Sheltron where it only works for a single attack. It can upgrade to the 4s version at level 35.

Warriors get Raw Intuition, but the self-heal on weaponskills doesn't come until level 56.

Gunbreaker get Heart of Stone, but it can only be used on themselves. Maybe reduce the damage reduction to 10% until level 68, where it gets the targetable action and increased reduction.

Dark Knight... is harder, because The Blackest Night really only works because of when you get it at level 70. However, Endwalker introduced another quick mitigation skill for Dark Knights, Oblation. Make it so that they get a single Oblation charge at level 15 and then they get the second at level 82.

There, now all of the tanks have a quick mitigation skill to help them understand that it's very appropriate to use them for tank busters since they come back up so quickly. Rampart and their 30% mitigations are more useful for long periods of incoming damage, i.e. large packs of enemies.