It’s possible to choose the line spacing by default so that its parameters are applied in every opened document?
No (unless you write a macro that would modify every opened document).
Documents that LibreOffice opens usually (except for most basic formats like plain text) contain all formatting information inside. LibreOffice is not a “reading” application which goal could be to present text in the form most comfortable to reader; it is a document authoring software, which goal is to retain all original data, structure, formatting of the document which it contains. If a document defines a specific line spacing (and any complex document format, like ODF, OOXML, or binary MS Office formats do contain that, even if in form of using defaults defined in relevant standards/specifications), then LibreOffice must use that value, not some user setting.