If you want to get a little deep into Alsa… you’ll need to load dietpi or Raspberry OS… something that has full access and Alsa support…
Then you can use alsaloop to send the Roon stream out of the bluetooth module on the Pi. Bit of a mess but I did get it working a long while back. Such a mess I never documented the BT side but here is how to get alsaloop working
A BT dongle may show-up as a “DAC” and, at that point, it’d just be another endpoint in Roon.
However, as other poster said… skip all this. BT on a Pi is a bit of a PITA. Just load Roon on your phone, pair headphones to phone, and use phone as endpoint. It’s far simpler and more stable and may sound better too depending on what BT CODECs your phone supports.