Unfortunately, there doesn’t appear to be a way to set a default line spacing.
However, a workaround is to open a blank document and insert (Draw) a
Text Box.
When clicking within the Text Box, you can set the font, font size, font colour etc, should you want, and change the paragraph line spacing to your preferred setting.
Then you the r/click the Text Box border and opt to save as a
Building Block with a suitable identifiable name.
In future documents, you then simply opt to Insert > Page Parts > Locate the created Text box and the formatting you previously opted for will be recreated in the Text Box when you add content.
If you have copied text, r/click within the Page Parts Text Box you have inserted and opt to
Keep Text Only (T):
. . . so the pasted text uses your preferred formatting (line spacing) created earlier.
Note: I'm using Publisher 2016