You may need to tweak the number here, but you'll want to create a custom cleaner and use a Regex Find Replace Action
With a 10000-character limit, I used 10 fewer as the number, with the assumption that any completed word would be less than 10 additional characters. You can lower this number to 9950 or even 9900, depending on what threshold you want for the breaks.
What it does:
It matches 9990 characters (. = any character). After that, it matches one or more word characters.
Then it "replaces" it with the match + the break text.
I tested it with lower character counts, and it seems to work as expected, although there may be edge cases.