I agree. From keeping logs over the last few years, severe depression was more likely to occur due to e.g. not enough sleep or just illness, and in those cases accomplishments were impossible. Rest was it, and I felt lucky if I could get good rest before acting on any depressing thoughts or ruminations. So I wouldn't call goal-reaching a standalone cure; far from it. Also there's the fact that people accept/reject potentially helpful information based on different criteria, from very rational (is it in a reputable journal) to irrational (do I like the person who gave the information) to incredibly subjective (did I think of it on my own?). So the idea of "why not try accomplishment" may be doomed from the start, due to factors outside of the advisor's control.
I noticed that rest only somewhat helped, but was significantly more effective if absent of any worry of external responsibilities.
E.g.
If I took vacation from work and didn’t tell my girlfriend about it so she wouldn’t worry about me and text me incessantly... I could turn off the thoughts/worries of the outside world completely for a few days and that made a pretty dramatic difference. I didn’t cure me at all, but it took me out of the really dangerous zone after about 2 days.
Also, sleeping in a hammock, haha.
So much better than breathing stale air for 2 days.
This is more circumstantial however, as my trigger at the time was accumulating too many perceived responsibilities and fearing failure. To be honest I feel the triggers are nearly irrelevant, but removing myself from them gives me enough temporary relief for my body to get out of stress mode. Which in turn removes some of the depressive overhead to give myself a golden window of opportunity.
