I can relate to the intuition that our actual motivation to cause SWB for other minds is strongly modulated by our empathy. (That said, there are also intellectual philosophical forms of reasoning, however I think they are practically weaker to motivate actual actions)

If I was severely depressed, my power to imagine happiness would plummet and I'd gain nothing from giving resources to a utility monster because the well-being it'd convert the resources to couldn't flow back to me.

Ironically, it's the opposite for me: My depression has increased my desire to see a world that is generally more good than bad, and my idea of good vs. bad reduces itself to hedonistic states mostly, because other values seem more symbolic than "real" to my intuitions (most of the time).

However, the good news is that none of us need to gain strong enjoyment from the modeling of utility monsters. If the path I outlined is realistic, then no step needs much self-sacrifice. A very small fraction of income donations of millions of mildly motivated altruists over several hundred years could cause far more SWB-over-suffering than has ever existed before in nature or human history!