To play Rosalina and Luma, you have to be aware that Luma is also there. You need to work with Luma and adjust your spacing. With Rosalina and Luma, you also have tools to juggle your opponent. Rosalina's Aerials are great, and you should use them often. As for land attacks, Up Smash, Down Smash, and Neutral Attack has the most use out of Rosalina's land moveset. If you need quick attacks to pull out, the tilt attacks are pretty decent and work better with Luma rather than Rosalina I think. Rosalina is very floaty, even when fastfalling, so watch out. As for specials, Neutral Special is the one you'll be using the most, as it separates Rosalina and Luma, which can set up attacks and combos. Side Special is a useful spacing tool, where Luma shoots out Star Bits. What's so great about this is that they can't be reflected or absorbed (I did a bit of research and it turns out, they're disjointed hit boxes!) It is kinda slow and leaves Rosalina in a very vulnerable state, so only use it for spacing. Down Special, like you said, can attract items towards Rosalina
What you didn't mention, is that it can attract projectiles too. If you use Down Special correctly against projectiles like Fox's Laser Gun or Samus's Missiles, you cannot get hit by these, giving you an opportunity to attack. It's also quick, which means you should practice avoiding projectiles with this move. Rosalina and Luma are largely defensive, where Luma protects Rosalina and Rosalina protects Luma. Kinda like the Ice Climbers, but if Nana actually tries to help you. There's a lot of risk involved in playing Rosalina, as she's only good against characters like Isabelle, Villager, or Wii Fit Trainer. But that doesn't mean you shouldn't keep trying!