@maxplathan, my pleasure!
This is a good framework, but might benefit from a few tweaks.
Unlike traditional graphic design, I don't "work to a brief" as a UX designer. Rather, I collaborate with clients and teammates to identify user pains and problems and agree on a proposed hypothesis/solution. The output of this activity sometimes results in a one-page project brief to get everyone on the same page, but it depends on the client.
I also don't think it's necessary to show the old website design unless the result is a drastic, easily-recognizable improvement.
Regarding the story, keep it short and sweet. I like to intro each slide with a short paragraph (3 sentences max) describing the problem, how we came up with the proposed solution, and the results. I realize that keeping it short with that much info is a challenge, but force yourself to be as specific and succinct as possible. If someone is viewing your portfolio online without you there to walk them through it, they will need as much context as you can give them.
I can't really tell you how many pages it should be. The most important aspect is clarity of communication. People will keep reading if the material is focused, engaging and they think you can help them. Also, getting feedback from friends/colleagues before you send your portfolio off into the wild will also help you gauge the appropriate length. Hope that helps!