I just like to summarise what how I setup my system today, as it is the only way that it seems to work (for me anyway :)) to save others time:

1. 861v8 L/R Master SpeakerLink Out to 218 SpeakerLink In
2. 218 Master SpeakerLink Out goes to Left Speaker (usually this is set to Master)
3. Left Speaker (Master) SpeakerLink Out goes to the Right Speaker (Slave)
4. Right Speaker (Slave) SpeakerLink Out goes to the 861v8 L/R Slave SpeakerLink Out
5. On the 218, select an unused source and change that to Sooloos. Rename all other sources to match it as per 861v8, set all of them to SpeakerLink. I have set all Comms Type to 4C but not sure if that's necessary. Change all unused Sources to Off.
6. Set both Controller for 861v8 and 218 to Auto

Everything works perfectly, and the only additional step is that you need to press "Off" before switching from 861v8 to 218 so that the 218 takes over the controller. I have my Roon via ID41 on one button, and I have another that will run through 218. So, I can do Trified when I am listening non MQA material directly through the 861v8.

I have tried other configurations, but the only way I can get it working is to have the Left Speaker daisy chaining to the Right Speaker otherwise it will lose communication. Neither the Right Speaker connecting to Slave of the 218 running Slave back to 861v8 nor connecting to 861v8 and have the 218 Slave back to the 861v8 works.

I haven't got a chance to run Room Correction again yet, but I guess that would be the last step if you want to align everything again.

MQA through the 218 definitely has an edge over the 861v8 with Roon setup as a Decoder. However, the difference really is minimal but I guess that would really depend on the source material.

Lounge: 861v8 with ID41, 218, UHD722, Sonos Connect Wyred4Sound, AppleTV, Oppo 105, DSP8000SE, DSP5200SE
Streaming: Roon/Spotify
Currently not in use: DSP5200 x 2 pairs, DSP5200HC, DSP3200, 218 x 2, MS600, DSW x 2, MC200, Sonos Connect Wyred4Sound