the more I understand Lady Russell and why she persuaded Anne not to marry Frederick.

I love Frederick. He is my favorite literary hero, but the young Frederick was too eager to take risk and, IMO, not yet ready for the responsibilities of a wife and (most likely) children. As has been said, he never saved a penny of his winnings, so what would Anne and their children have lived on if anything had happened to him?

Anne, being the daughter of a baronet, is accustomed to a certain standard of living. I cannot imagine her, as a young bride, living like the Harvilles do (or the way Mrs. Smith does), in drastically reduced circumstances.

We also need to remember that Anne cannot just go out and get a job to help support the family. If anything happens to Frederick, she runs the risk of being destitute and, given who her father is and what his attitude was towards the marriage, she would have nowhere to go.