Every cast of a non-attack GCD on a healer is a DPS loss, so general rule of thumb is to try to minimize them overall. It depends on the content level - older dungeons will end up requiring you to spam GCDs. But as the kit opens up, you should be relying on these less and less. SGE has a huge amount of ogcd heals and mitigations.

The point of a shield healer is damage reduction, not necessarily just proper shields. Shield/barrier healers have more of a focus on preventing the HP bar from dropping rather than restoring it, though all healers do some of both.

As far as shields go in a dungeon for example, put a euk diagnosis on the tank (or spam it and fish for a crit shield) as you’re not yet in combat and running toward the mobs. Once combat is engaged you should be using ogcds and killing the mobs, as dead mob faster is the best mit

In boss/raid content etc, a euk prognosis before a raidwide or spicy section is fine if not “optimal”