A very simple solution would be to make the rank1 spell get a potency gain up to the current rank2.
So "new Cure" = "Current CureII"

Upgrade rank2 spells to even higher potency and slap a 30s CD on it.

This way both rank see a use. The current "use the highest rank" wouldn't change, but since it has a 30s CD, you simply can't spam it. Basically like an ogcd but with a cast...

Regarding SCH, emergency tactic already work as a gatekeep. In order to match the new design, Physick would get a potency boost and Emergency Tactic would gain a "Increase Adlo healing by 25%" or something to match the desired raw healing.

Sage could simply have a CD on Euk-Diagnosis.