I find it interesting to hear you say this, to me, partner feels like quite a warm term. I think probably because for me, I associate the word partner with a sort of "levelling up" of a relationship, where you're not just two people dating, but two people in a partnership to help each other be the best they can be.

I do agree that it feels clunky sometimes though. I was catching up with a friend recently who paused just before the word "partner" every time. I pointed it out and we laughed at the relatable awkwardness of not feeling like you have a correct word when "girlfriend" doesn't fit. He said that partner didn't feel right either, but it was the closest he could find.

I don't recall ever hearing significant other irl, though I do sometimes see it online as SO. To me, that feels more clunky than partner.