I don't know well about how to use the expression "more of ~".
That is a great question. "More of" here is being used in a similar way to "more to". He's saying that he believes that their future has "more to" it than just making these jars of jelly. We use "more to" to signify that we believe there is something more interesting, or impressive, or complicated that to an issue. In this case, he thinks his future should be more interesting or impressive than just making jelly. I do not think this phrase "more of" is a very common one and you're much more likely to hear "more to".
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