devtrackers.gg
I don't know how new this is, but I found out that there are checkpoints inside the story mission so if you don't finish the instances in one go, when you go back all your progress is saved. I had to stop playing yesterday before the final boss of the first mission and I come back to find I'm outside the final boss room. Just wanted to make a post so people know because I sure didn't.
EDIT: To add on to this, yes this can be used to retry achievements, just relog on your character.
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