It beats doing nothing. Vegetables are harder to avoid eating at all, but fish? You can totally do without eating any ever again and be fine. And almost all salt you can buy in stores has iode added so you won’t miss that nutrient.

Only if eating ethically is defined binary, not on a gradient.

A huge portion of harm can be reduced/prevented with only a little effort. Don't let perfect be the enemy of good.

Not entirely true. Buy produce from your local organic farmer at the farmers market and you'll probably be fine ethically.