Before choosing these MusicCast yamaha units I would make sure you are comfortable with the uses and limitations of MusicCast.
https://forums.audioholics.com/forums/threads/seeking-system-ideas-for-multi-zone-audio-1-video-zone-older-home.97509/
There is a thread with some useful info from a previous discussion. Also look for a local dealer that sells Yamaha AVR's and go in and ask for a quick demo of how MusicCast works. There are also some youtube videos you can watch that give you some idea but be wary of marketing hype in the yamaha released ones.
One thing to note is that bluetooth is the old way to cast music to speaker systems and has many limitations. It drains the battery of the phone streaming the sound and has range issues and drop outs if you move around to much. Can also be fiddly to pair and switch between multiple speakers. solutions like MusicCast and ChromeCast Audio have some advantages over bluetooth.
Also the RX-A850 has some limitations when running multizone. It can only power 5.1 speakers in the main room if you use it to power zone 2 speakers. Can add a stereo amp for the second room to keep 7.1 in the main room though. Also it won't pass HDMI or digital audio out of zone2 as it only passes analog, radio, bluetooth, USB or network sources to zone2. I'm not sure how well the zone2 works with MusicCast as I have never used one of these units to try it out myself.
Another note is that you can join an external bluetooth sound device (like the Pyle amp you mentioned) to MusicCast as an output device but their may be some fiddly setup for this and there is audio delay issues. Also extra bluetooth output speakers added to MusicCast do not magically give you extra 'Rooms' to feed audio to as they are somehow tied to the main room you are casting from. So they work more like extra or alternative speakers for an existing room setup.