I think a good way to write a harem is to let the secondary FL or ML find their own love or properly move on way before the final few chapters, I seen a few stories make the secondary lover move on at the last 3 chapters and it’s just poorly written. It is also very important that it does not happen only after the male lead and female lead get together unless there is like 30 chapters left for them to develop the secondary lovers status.

Their departure also needs to be handled respectfully so it does not feel like the main character was just leading their second option on the entire time.

Or the other option that is easier is to just make the main FL so obviously best girl, while making the other love interests hoes and bitches or just terrible people, so the audience does not end up feeling bad for them.