How do I stop polishing work after it already meets the brief?

Your manager gave you a useful definition of this version: checked figures, a clear argument and enough structure for review. I’d put those requirements beside the file and send it when they’re met. The heading could be improved later if it still matters. The immediate quality question includes whether colleagues receive it in time to do their part.

Sending a draft is also how you learn whether the argument works. Another hour of spacing changes can’t answer the questions your reviewers need to answer. You could label the version and say what feedback you need, rather than attaching a general apology for it being unfinished.

Which small changes are genuinely making the pack easier to use? If a heading is confusing, fix it. If you’re switching between two equally readable layouts, the decision may not need more time. Naming the specific benefit could help you distinguish a useful edit from an edit that mainly postpones sending.