But these clients have already expressed a desire to invest in startups. If your idea of working in their best interest is to "not invest in startups" then you shouldn't take the capital (and fee$) in the first place.
I agree that if everyone is being ethical then that is the right thing to do. I'm just worried about the situation where a fiduciary might actually be doing what they think is best by waiting for a better opportunity and I'm not sure how you judge whether they are doing that criminally or sincerely.