If OSS project maintainer offers “fixes for money”, then could there be a conflict of interest? Merging a PR that scratches some obvious itch would be equivalent to robbing my family of money I could earn by charging a BigCo and doing it myself.

IMO one of the best ways to contribute to OSS while making some living is to do product development and/or consulting work, and structure it so that you can methodically encapsulate and open up the components involved. Of course, this unfortunately does not apply to every project out there, but I believe Django was born and had been developed this way for example, as well as probably many well-known projects that we don’t know the precise origins of.