Sharing my most recent experience.
My fiance is in the US and I'm from Japan. I have an existing US travel visa and has traveled in the US before. I was a bit anxious of visiting in general. I prepared all the suggested documents, proof of house rental or ownership, employment certificate, and bank certificate.
When I entered the US, I told them Im visiting my boyfriend. I didnt say fiance. Officer asked if we were engaged. i said yes. He also asked when we plan to get married, I said we havent set a final date yet and still planning.
After that, no major questions and he just said welcome back.
In general, if you have good travel history then it's not going to be a problem.
Some tips:
1. Even if you have traveled to the US before, just be extra prepared with supporting documents. I didn't have to show it but it gave me peace of mind.
2. Do not overshare. Just answer the question straight without volunteering info that is not needed.
3. I think it pays to dress properly. This is just me. But i wanted to look professional to create a good impression.
Hope that helps!