From what all I've read the over-looked (under-popularized) factor in depression is INFLAMMATION in the body. Diet can have a huge impact on this and in general the inflammatory foods (vegetable oils, sugar, processed carbs) can contribute to or cause depression, and the anti-inflammatory foods, like fatty fish (wild salmon), greens, etc can help.

Yes and inflammation may be related to the microbiome too.

Lists of foods and their inflammation levels are easily available on the internet, so talk of "hiring a dietician" is overblown.

Lots of stuff is on the internet. That is not a substitute for consulting a professional at least not for everyone.