Long end of 100-400L Zoom & 1.4TC & ca. 100% Crop and ISO 1600 is the limit for the R6, I think. The second one is ISO 12.800 and you have all the artifacts and grain etc.
I absolutely adored my R6MI for the two years I had it. I recently upgraded to the R6MII and honestly, as much of a beast as that camera is, I miss my MI sometimes There's just something special about it.
Most of the photos on my San Antonio Wedding Photographer website were taken with one of the two. Here is a recent photo taken in Downtown San Antonio between mini sessions with my R6mII and 50 1.2 L RF lens.