Question for experienced tanks.

[ItMe]

My question is simply how demanding is this role for prog content?

Tanking is straightforward enough that most tank mains I know are also the primary person doing callouts for their team.

Is it a very hard to master/play role compared to a dps class ? [...] is it forgiving to rotational mistakes?

The rotation is far simpler than any of the DPS jobs, so you're able to focus on what you're doing much better. It gives you ample opportunity to pay attention to what the boss is doing, as well as stay firmly on top of your own responsibilities and help cover for any mistakes.
I personally find it to be easier to master than DPS classes, but I tunnel vision as most DPS so take that with a grain of salt. Tanks being able (and asked) to focus more on what the boss/enemies are doing is really helpful for me.

Yes, it is quite forgiving to rotational mistakes.
For starters, because the rotations are pretty short.
And if you do mess up your rotation, worst case scenario you miss out on some DPS or some resource generation that goes to more DPS, which is important but as a tank that will rarely make or break your prog.

And what is the best ways to practice being a tank in said content,

The best way to practice being a tank in said content is to just do it.
Go into whatever level of content you're comfortable with, once you acclimate consider doing something harder.
There is a general difficulty curve from beginning to end of each expac.
So if you're looking to tank EXes, the two you get through MSQ are excellent places to start.
If you're looking to tank Savage, the first tier is always designed as a bit of an introductory one and a perfect place to hop in.
And if you're looking to tank Ultimates, I'd probably recommend starting with UWU.
But you don't need to wait until 6.0. Especially with the 2 week delay you can get your feet wet tanking Innocence EX or Eden. Being quite over geared will probably help quite a bit if you're nervous, so give it a shot.

Thank you very much !

Have fun and good luck~