I don't know much more than you could find searching around r/nootropics, but my sense is that the relationship between diet and cognition is highly personal, so experimentation is warranted. Some do best on keto, others as a vegan, etc. With respect to particular substances, it seems that creatine might have some cognitive benefits, but once again supplementation is highly personal. DHA helps some people and induces depression in others, for example. Also, inflammation is a common culprit/risk factor for many mental issues, so I'd expect that a generally "healthy" diet (at least, not the standard American diet), and perhaps trying an elimination diet to see if eliminating certain foods produces a marked benefit could be helpful.  Supplements like resveratrol might help with this as well. Also might be worth experimenting with fasting of different lengths; some people find that they become extremely productive after fasting for 12-14 hours. There are a million studies on all these topics that will come up in a Google search.