Have you been doing the exercises from the workbook?
The words aren’t archaic, but they are often bookish. That’s because jukugo (kanji compounds) tend to be used more in written language than spoken language. It’s a natural side effect of WaniKani being a kanji learning site.
The most important thing is to apply what you’re learning. If you want to get better at forming sentences and speaking, you have to do those things often. That means being strict with yourself not to fall back to English when talking to your girlfriend and perhaps asking her for feedback (if she doesn’t mind).
From the consumption side, book clubs also help solidify grammar. Only being partway through Genki 1 is a bit early, but you could always join one of the book clubs here on the forums.