Castform is a lie, I've decided. I can't seem to find a good spot on that route to get rustling grass with any consistency, and his 5% chance seems like a 0.1% because it's always something else. I've skipped him for now, after looking for an hour or 2 the other day.
When you get sad, stop being sad and be awesome instead.
Feebas is Emerald, 6 tiles out of 500 (or some ridiculously large number like that) 5 casts in each spot, about 2-3 hours a day fishing. But then how about you find one, weaken it then make it faint, and you're out of pokeballs, so back to town, all tiles reset, repeat.