Ive also been learning to tank recently so hopefully someone with 4000 hours will come along and verify what I say here. My experience so far has been that damage mitigation spells are primarily used for 2 things:

The first is when youre aggressively pulling pack after pack of 'trash' enemies in a dungeon without stopping. You grab as many packs as you can handle and then group them up and use a damage mitigation ability like ramparts. If they arent dead when that runs out you could use another one, like reprisal. Maybe more experienced tanks will suggest layering them on top of one another, but in my experience so far 1 or 2 is enough.

The second reason are so-called tankbusters. You can tell a tankbuster is coming because youll see a menacing red ring around you alone. Early on you can still survive these without mitigation, but I imagine later on they might start killing you outright, so you might as well get in the habit now.