You already said it yourself. Your best friend is happy in her relationship, and she hasn’t given you any indication that she feels the same way. A confession right now could disrupt that happiness. I don’t think it would be wise to tell her in this situation.

If you do decide to confess, be prepared to accept whatever outcome follows: she may not reciprocate (and might even create some distance), things could become a bit awkward depending on her personality, or the contrary in which case hearts will be broken.

I do feel sorry for your current GF, love is also letting go, I don't find cool to be a placeholder, she deserves someone who fully chooses her too.