You could try Dmaths (see a demonstration video), a free software extension for LibreOffice. Formulas are an object in LibreOffice, so they will be not, to use your own expression, "truly inline".
But if you are not afraid to learn something new (but LaTeX scares you), I would definitely recommend GNU TeXmacs. The files will not be in a format that LibreOffice or MS Office can recognize, as far as I know, but you will be able to type quite simply all the math you want, and more. GNU TeXmasc is a WYSIWYW (what you see is what you want) editor.
LaTex is still the best, in my opinion, to create beautiful documents. If you can't wait to see the output of what you are writing, you can (almost) do it if you edit your documents with LyX. LyX is WYSIWYM (what you see is what you mean) editor.
LibreOffice and MS Word both are WYSIWYG (what you see is what you get) applications.