As a habitual 'big hps' healer, I definitely feel where you are coming from, but IMHO it adds to the flavour of the job and frankly is a necessary evil to keep our throughput potential in check to some degree. It's also typically only ever an issue within the first 2 to 3 minutes of an encounter when it is an issue at all, if I pull agro beyond that point, chances are somethings going horridly wrong. A12S was a bit of an outlier with how long the boss was untargetable for but it was rarely an issue that couldn't be worked around there.

In your case in particular, I'd advise your WHM to hold their Lucid Dreaming a little longer. It looks like they are using it on the Ultima just after the first Celestriad. I have better luck using it 15 seconds later just after a Cure III following the first Forsaken. This gives the tanks the biggest head start on enmity without sacrificing my MP too much and means my second Lucid is popping a few moments before the second Forsaken after Trine etc.