It looks like your formula is relying on the Autonumber to come up with the sequential numbers. So your ask isn’t really to adjust the formula - but for autonumber to work differently. Autonumber is pretty dumb. It only does one thing - when a new record is created, the number goes up.
Unless Airtable adds features to autonumber, use an integer field. That’ll give you the ability to assign whatever sequential values you need. The big drawback of course is that it’s a manual rather than automatic assignment. So if you need to assign a whole batch of numbers, you can fill in the first couple rows (for example, 1 & 2), shift select both of those rows, and drag the small square down to automatically count upward.