The usual method would be to create a template as desired in Writer and then create documents using the template. Maybe there is a good reason to go through the extra steps of exporting it to pdf and merging the pdfs but it sounds a bit like printing documents on letterhead paper like we did in the 1990s.
I made a couple of documents that could be used as templates in Writer. I don’t have your BauerBodoni RomanOsF font so I made a substitution in LibreOffice for Bodoni BT which is not the same but close enough for me.
The first template will use BauerBodoni RomanOsF if it is on the computer. The template uses First Page style for the first page, second and subsequent pages will be in Default page style which may need editing to suit. [Edit: modifed template]
b_letterhead126034EA2.ott (84.9 KB)
The second is just a document, not a template. In this I copied the layout (with Bodoni BT font, this will not change on a different computer) from Writer to Draw, saved the page as .svg and inserted the svg as a background for First Page page style in Writer.
LetterheadAsBackground.odt (89.6 KB)