FFs are generally deficient in all nutrients that frogs need. There are some fairly complex considerations around trying to boost levels of any of those nutrients through FF media, which can be researched with a very deep dive in the archives; I don't even know exactly where to point you, and it is pretty rocket sciency stuff that isn't going to yield any measurable positive results for a casual keeper anyway.
All that said, there are serious advantages to using a quality prepacked mix. One is that they're very consistent, and so any results can be expected to continue. Another is that they're used by many keepers, so troubleshooting FF production with community input is a lot more practical. Perhaps most importantly, if a keeper has any relevant problems with their frogs, a DIY mix will not be able to be ruled out as a contributing cause and so will make troubleshooting those frog problems much more difficult.