Styled Text Editor

Here is a sample code where text length validation is kept at 20 characters. Hope this help for your case

a!localVariables(
  local!showFullText: false,
  local!text: "Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum",
  {
    a!styledTextEditorField(
      label: "Styled Text Editor",
      labelPosition: "ABOVE",
      value: if(
        and(
          len(local!text) > 20,
          local!showFullText = false()
        ),
        left(local!text, 20),
        local!text
      ),
      saveInto: {},
      validations: {},
      readOnly: true(),
      sizeLimit: 4000,
      height: "MEDIUM"
    ),
    a!linkField(
      links: a!dynamicLink(
        label: if(
          local!showFullText,
          "Show Less",
          "Show More"
        ),
        value: not(local!showFullText),
        saveInto: local!showFullText
      )
    )
  }
)

That's difficult. I think the markup is pretty simple and uses P-tags for each line. So you could find the next closing P-tag and truncate the text after it.