Your Idea is better then the other with having all the abilities in lower dungeons, but even then i see some problems:

DPS: They worst DPS (for combos to use and as far as i remember) for Sastasha has a 1-2 single target combo (Dragoon) and it goes up to a 1-2-3 1-4-3 combo for monk. And for the first AOE, thats soemthing i wished for a long time.

Healer: For AOE, i agree, but the second effect (Stun from Holy) should be deactivated. But you seem to have not played Healer in a long time. You have Rescue and Resurrection even in Sastasha. The mitigation is in theory good, but outside of full pulls from the tank it is not needed in the ARR leveling dungeons.

Tank: They have at least 1 Mitigation even in Sastasha, probably even more depending from what level they sync down (remember, all role actions are avaible on sync down) and they get the aoe attack bevor they even can enter Sastasha for the first time. About the death denial, that is outside of full pull not needed.

But the idea to have at least something better is a good idea. The problem is they had in the past a better feeling in the ARR Dungeons, but they lost this with the Expansions when they removed abilities to give the jobs new abilities and i don't see them giving it them back.