A last update: I never got my R2 to work correctly with the Galaxy s9+ running Android oreo (8.0). Trimble support insists that NMEA is not necessary, and that their GNSS Status/GNSS Direct apps handle the translation between the R2 and Collector (classic). I could not get it to work, and Trimble Support just kept insisting via emails after several day delays that it works. I was able to connect to the R2 with GNSS status, set up corrections and get a good position. However, after setting the location provider to the bluetooth R2 in Collector, GNSS status would lose it's connection to the R2 as soon as you open the map in Collector, then you get no position in collector.
It does work using NMEA as I previously described, but using this method you can not receive corrections via NTRIP, or presumably any other non-SBAS correction method...that just makes the R2 a really expensive R1.
You can also use mock locations to send the corrected location to Collector, but in that case the receiver name recorded in the feature service metadata is incorrect- the devices integrated receiver (in my case :Samsung...) is listed instead of the R2. Neither of these were acceptable solutions for our workflow.
Ultimately, I ended up abandoning the Android device and am switching to Apple devices. I've tested on the iPhone 8 and it appears to function as promised. The new Collector app handles corrections internally, so when that is fully functional it will probably be the best solution, and ESRI appears to develop for iOS before Android.
Trimble Support wins no customer service awards through this process-In several communications they didn't seem to even know the specs or operating procedures of their own equipment and software. If you're taking this on, be prepared to figure it out yourself.