I have healed almost every single content in this game since 1.0 (only current patch, Coil Savage and Ultimate excluded) and I have never ever met a tank so bad I would have not had any time to DPS. Of course there are situations which require a lot of healing, but looking at standard runs rather than exceptions, it is clear healers should have plenty of time to DPS in all content.
I don't know if you've played healer in this content yourself, but in many low level dungeons you don't really need to cast many heals at all during pulls. In Sastasha you mentioned as an example, I once watched my partner heal it in full-time cleric stance (pre-SB) using nothing but Essential Dignity.
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