Is there a way to do iterative rendering modifying state and reading the final page counter?

Questions
2

If I’m getting the gist of your question correctly, you want to know whether some content can fit in a single page, and adjust the content until it can. I think you can achieve this in Typst; here are the fundamental steps:

  1. Put the content in a function that you can call with different actual-thresholds. You don’t need state for this; just do something like this:
    #let full-content(actual-threshold) = {
  let item-with-threshold(level: 0, ..args) = {
    if level <= actual-threshold [#args.pos().join()\ ]
  }
  item-with-threshold(level: 0)[This item has a very high priority]
  item-with-threshold(level:1)[This has lower priority]
  // ...
}
  2. Use layout and measure to determine the height of the content on the actual page:
    #layout(size => {
  let measured-height = measure(full-content(2), width: size.width)
  // does it fit? check `measured-height < size.height`
})
  3. Use binary or linear search to determine the largest actual-threshold that fits. Do all this inside layout(), or you’ll run into Why is the value I receive from context always content?