The general suggestion is to organize files (modules) by a central type. My approach is also to keep everything in a single file until it feels too large and then I’ll move 1 or 2 types out into their own module (along with related functions).
There are packages, like dillonkearns/elm-form, for helping with forms. If one of those packages doesn’t fit your needs then I think I’d need a little more details about what you’re looking for. I’m not really sure what a form for a Page Model really means in this context.
One advice would also be to not overthink it at the start. In many languages you need to get your architecture “right” from the get go. In Elm though it’s usually very cheap and very safe to refactor your code in even the largest ways.
So if that’s something you’re comfortable doing, i would suggest you just start out and put something down within the very basic elm architecture. Do not look left and right at packages that might help because they might also just confuse you even more. The very basic elm program is easy to get into simply because the architecture and the code is so self-explaining and explicit.