You can change the default settings for Spacing Before/After and Line Spacing in the Paragraph dialog box. When you are done, click the Set As Default button. Later, if/when you are prompted to save your template, choose to do so.

However, as Charles and Guessed have said, the key is to make proper use of styles in Word.

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Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional