This is what I hoped I could do, but my understanding was that the Roon app was only for controlling playback to my existing endpoints (e.g. my RoPieeeXL). This post seemed to confirm that, and I’d never seen an option to actually play music THROUGH my iPhone, only to use the iPhone as a Roon remote. However, I just found this post, so it looks, so I guess I should be able to play via my iPhone after all (and then stream to any BT device I want).