Can anyone help me with a formula? Here’s what I want to do:
Take a total number of words: for example, 60,000
Subtract my current number of words: for example, 20,000
And then take that number (40,000) and divide it among the number of days left before my deadline.

The number of days left before your deadline is a formula field that displays a string of text, not a number. You need a formula based on the original due date (date field) and calculate the number of days remaining as a number, not a text string.

DATETIME_DIFF({Due Date}, TODAY(), 'days')

Then subtract and divide as you described.

( {Final Word Count} - {Current Word Count} ) / {Days Remaining}