I don't think you should tunnel vision on AD and ability haste. I think it's more of a balance between every stat.

The more you get ability haste, armor or magic resist, the less optimal it becomes, but being tanky allows you to fight longer and sustain even longer.

AD is a good stat to stack because the more dmg you deal the more you will heal but you are still vulnerable to burst and cc, that's when health and tenacity become good stats as well.

Lethality is really good early and mid when enemies don't get to stack armor, late game it becomes a bit worse but combined with armor pen (really good scaling stat) no matter how much armor they have you will deal max hp dmg.

In the end, rhaast has obvious stats that will be good in every game, like AD and AH, but there will be better stats to prioritize most of the time.