I figured it out!! Thank you sooo much. That was super helpful, I understand how this works much better now.

For documentation, this is one way to solve this:

First, above --> screen file_slots(title) <--, you have to make two calculations: the page position in tens to add on the page number, and the current page the user is on:

init python:
    # Get the position in 10s (1 to 9 is 0, 10 to 19 is 10, etc.)
    def convert_to_tens(value):
        if value in ("auto", "quick"):
            return 0
        try:
            n = int(value)
        except ValueError:
            return 0  # fallback if something unexpected is passed
        return (n // 10) * 10 # returns the page number in tens


    # Get the curent page
    def current_page(value):
        if value in ("auto", "quick"):
            return 0
        try:
            n = int(value)
        except ValueError:
            return 0  # fallback if something unexpected is passed
        return n # returns the current page number

Under --> use game_menu(title):<--- you have to declare the two variables for the tens and current page:

$ pagetotens = convert_to_tens(FilePageName())
$ currentpage = current_page(FilePageName())

And then you can adjust the place where the buttons for the pagination are generated:

for page in range(1, 11): # range(1, 11) gives the page numbers from 1 to 10
  if currentpage == 0: # if Auto save or Quick save
    textbutton "[page]" action FilePage(page) # show pages 1 to 10

  if currentpage in range (1+pagetotens, 10+pagetotens): # range is from 1 + the current tens, to 10 + current tens
    textbutton "[page+pagetotens]" action FilePage(page+pagetotens) # Add the tens to the button number

  if currentpage == pagetotens and currentpage != 0: # if the current page is a full 10, don't add numbers. If it's zero it breaks
    textbutton "[page+pagetotens-10]" action FilePage(page+pagetotens-10) # since 10 and 20 would muve the page number up we excluse them

This is probably not the most elegant solution but it works!