tldr

This is how I do it:

On all heals I got
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On my casted damage spells and instants that do not trigger gcd I got
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On my instant damage spells that trigger gcd I got
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In healingmode I focus the tank and can toss some spells on his target but am unable to trigger a gcd with an instant. Just a saftynet - If a heal is necessary I can abort any cast while focusing the tank. The only exception I set for this is ruin II. I cast that way to often in healing mode to prevent incoming damage.

As the fight goes well I can switch to an enemy still being able to heal his target (most propably the tank) but also being able to dot em up.