I'm editing a rather long document that has various spacing for first line indentation. I need to standardize it.
Is there a way to do that without doing it manually for every single paragraph?
You can change the first line indentation by highlighting the entire document and using Paragraph settings.
Normally in Word, your paragraphs should all be of a certain style (maybe 'Normal', maybe some other custom name).
If you adjust that style, they are all adjusted - that's the idea of using styles.
It makes little sense to find a workaround to avoid one of the core functionalities of Word, and then make it behave as if it was used anyway.
So the strong recommendation is to change all your text (that you'd like to be affected) to a specific style, and then adjust that style.