Welcome to the Bunnings Workshop community @Buildabridge. It's amazing to have you join us, and many thanks for your question about building a bridge.
I've created the below renders to give you an idea of how you could construct this bridge. Please remember that the Matrix panels are not designed for walkways at height. If you are to use them, I suggest having your 90 x 45mm timber support them adequately at their mid-joins and where the panels meet along the length of your bridge. This means that at every point where you've clipped the panels together, they need to rest on top of supporting timber.
190 x 45mm for the side rails sounds about right, but you might consider installing some midway posts that are concreted into the embankment.