Hello! To crash higher chemistry without National 5 is doable — but you must put in at least twice the effort into the subject, meaning that you will need to spend less on other higher subjects.
Higher is, if not the most, important and stressful year of high school and so, in my opinion, I wouldn’t suggest crashing a higher in s5, instead do it in s6 (where you have less subjects and more time to revise each).
Hope that helps!!