I think the invulns existing as they are, is not a problem at all.

My reasoning:
In dungeons tanks should already be using their CDs on a rotation of sorts to help mitigate damage when you do large pulls, I do not see a problem with the current timers as I have been able to do this on every tank effectively. The only thing that's nice is Invulns give us the opportunity of a panic button or to make taking that damage more interesting, PLDs have the best one without taking any damage, the GNB's health goes to 1 but is invincible for 8s so plenty of time to heal up, and WAR and DRK can be avoided or useful to help situations - this is not meant to cheese anything but can spice up the gameplay a little bit.

When it comes to the ShB savage raiding and the recent designs they have done to them, as a tank, I do learn the proper way to do all the fights, it is entirely possible to mitigate or tank switch when appropriate - the point of the invulns in these fights is not for "cheesing" to avoid learning a mechanic, but to either make very complex parts easier or in the case of something got messed up, to save a run in the savage fight instead of having to rerun the fight over again. I think SE has done a great job in their fight design in introducing more difficulty for all roles, more responsibility of each role for the entire fight itself, while making it entirely possible for those interested in getting into savage content to learn and grow.

If the savage fights are considered too easy or simple for people, that's why the Ultimate Fights Exist. The invulns do not need to be removed or completely reworked when they are put there as part of the toolkit, finding inventive ways to use them IS part of the fun in my opinion. As a DRK main, I don't want to use my invuln to make it more difficult on my healers unless we have an understanding that - yes this will work - it was never meant to be a cheese other than making it easier overall for my party or healers or even my co-tank.

As for the CDs in savage content, in E5s I pretty much use them on rotation for the TBs of the main boss, but my OT does help when possible and I help my OT when possible. In E6s, other than conflag, i'm mostly free to use them as I take hits and cover my co-tank when they get the TB which makes it so I don't have to plan as much but the fight itself has more variety in its mechanics. E7s I do find to be the easiest of the latest tier. E8s everyone has responsibility, just different responsibilities, there is a needed tank swap - do some people cheese through it with their invulns? Possibly, but not all can, I am using my raid wide CDs strategically as well as making sure I use my personal ones at different points of the fight. Can these fights be done just using the CDs where absolutely needed? Sure, these fights were designed so a particular skill range can still conceivably clear content when put together and not just for the elite of elites.