The usual solution involving water temperature is to have levels of suitability.

I want to shower in hot water, not cold water. Absurd? Not really. Just simplified. In fact, the joy I will gain from a shower is a continuous function of water temperature with a peak somewhere near 45C. The first formulation just approximated this with a piecewise line function for convenience.

Carrying the analogy back, we can propose that the moral weight of suffering is proportional to the sentience of the sufferer. Estimating degrees of sentience now becomes important. ISTR that research review board have stricter standards for primates than rodents, and rodents than insects, so aparently this isn't a completely strange idea.