I use chunks of styrofoam as pot fillers (only for non-edible plant like hibiscus). Makes the pot lighter, plants don't seem to mind. Packing peanuts should be fine.

I would repot them when you want to encourage growth; i.e., next summer.

I'm bringing them inside I do want growth/blooms prune when they stop blooming but I want them to bloom! Thank you!!

If you do use packing peanuts, I highly recommend you enclose them in something like between layers of window screen or landscape fabric. The reason I say this is because it's no fun trying to pick those peanuts out of roots or soil later. I'm not a fan of the peanuts for that reason.

In the past, I also used sliced pool noodles in some pots in the bottom then covered with landscape fabric before adding the dirt.

However, this year i switched to using pinecones as a space filler. I find them more pleasant and also organic.

Nice idea suggestions, Popmama. I too have used packing peanuts and hated having to pick them out later when repotting.