it seems a lot of people find LongeCity while searching the term "nootropic"
It's complicated. There's certainly something about to be said about investing where the membership fees come from. But if they're already finding it using organic search, is there really a point spending extra on it? Maybe. This is best studied by experiment.
Likewise for YOLF's suggestion -- the "charitable" terms. I can't imagine any one seriously interested in these topics would not, in due time, find this site on their own.
So the way I think about Google ads is not in terms of finding people who are already searching me. But I try to think, who wants to know about me, but is NOT searching me directly, for various reasons, and so need to be helped by being shown the ad? (They don't know the right words, they don't know I exist? Something else?).