You don’t actually need to turn it off manually. Roon knows what the 818 can do and behaves accordingly.
Yes, ensure that it's set to "Decoder and Renderer". If you don't use DSP (including volume normalisation) then the MQA stream is delivered bit-perfectly to the 818. OTOH, if you want to use DSP, you get the first decode in Roon plus the DSP (but currently no subsequent unfold/rendering in the 818).
But I can say, as someone who uses room correction, that first unfold + room correction + no rendering sounds so much better in my room than full unfold and no room correction.
Hopefully soon, with firmware updates from Meridan, we'll be able to have our cake and eat it.