Hi @michael_maharam,
I’m glad to hear you’re up and running! Please let us know if you encounter any issues moving forward.
UPnP will not present a security risk in the context in which it’s used in ARC, provided you’re not demilitarizing an entire router or placing devices in front of your router’s security. Please see this post for additional context and discussion: Security with Roon ARC / Roon 2.0 - #24
As for your question, it’s absolutely possible to download your local library to your phone, depending on the local storage available on the phone and the size of your Roon database (library and Roon’s machinery together). One option is to add all of your content to a playlist (an “everything”) playlist. See this thread: Roon ARC - How to download all music? - #5 by brian2
However, note that ARC doesn’t allow for downloads of Qobuz or TIDAL content. Downloading and offline playback are limited to your own local library (music you’ve purchased, imported, or downloaded). Unfortunately, we may never include TIDAL and Qobuz downloads as a feature in ARC, due to licensing restrictions with offline playback on the part of the record labels, rights organizations, and our streaming partners. It has a completely different licensing structure than streaming.