[Quoth]

Never use cm for margins or padding for ebooks! That's for PDF output.

Use pt and most ebook software will regard 12 pt = 1em. Using em is best for margins, padding and fonts, but LO doesn't have that. So using pt is next best. So 18 pt is 1.5em and 16pt is a reasonable indent.

The Kobo kepub format uses a slightly different equivalent for the pt rendering, so any epub intended to be a kepub should have all converted to em for margins, padding and fonts.
The body font should be 12pt as that is 1em. The body font size ought to be 1em always, and usually is that if not defined.

Also don't set line spacing, or else have the conversion settings remove CSS line-height, because it's default is set by font metrics if not set and then the user on the reader gadget or app can change it. They can't if you set it in ebook.

docx to epub is fine using margins, padding and fonts. Using cm is meaningless for ebooks.

Images may or may not rescale with user font size on the reader GUI if using pt. You should use px for images, though large ones may need auto and %.
The Images css will need manually adjusted.