[Quoth]

If a document was very messed up with direct formatting I'd copy and paste to a text editor (Notepad++ on Windows, KATE or anything on Linux). Then open a new blank document and copy /paste into it.

Then edit the default paragraph style. Normally all four margins and all four padding should be zero, but a first line indent should be to taste. Outline level = body text

Next find the headings and select a heading paragraph style. Edit it to taste (the style, not the heading). Outline level 1, 2 or 3. Apply to all headings. Make sure no auto numbering.

Have the Style browser and Outline browser windows open.

Now you can click on any heading in the Outline and select the next paragraph (first after a heading). Give it a new Paragraph style with maybe no first line indent and a non-zero top margin. Apply that to all first paragraphs after headings and breaks (though a 3rd style might be used for those).

Only apply bold to headings and only in the style, never direct.

The only direct formatting EVER at most is Italic words, superscript and subscript.

Do not use bullets or numbers for lists. Instead create a list-like paragraph style and type in first character.

Don't use Web style block-quotes. Create a more indented paragraph style. A poem or song may need:
Optionally a very first line style
Verse first line style (adds top margin)
Verse body style
Verse last line style (adds bottom margin)
Optionally a very last line style