I think unless Canon has some surprise for us R6 @$2500 won't be much of a competition in photography to Sony a7 III/a7R III, due to higher price, lower pixel count and worse lens selection - unless R6 has something special in terms of image quality, looks like Sony will be the clear choice.
It is aimed at the future A7IV (or a bit towards the A7SIII as well).
It has much better video specs than the A7III with 10bit 4k60p, swivel screen, probably better IBIS, more advanced AF, way better EVF and screen, both card slots are UHS-II, etc.
A7RIII is a cheap option to get 42MP, but severly lacking in other aspects, slower frame-rate, not as good AF, no full touchscreen, etc. I mean it is a much older camera of course, so it should be lagging behind.
And while the current lens selection is less, but what they have for the new wider RF mount is looking better than the Sony, with IS on both the 15-35/2.8 and 24-70/2.8 lenses, with Sony one has to go to f/4 to get lens IS, 70-200/2.8 much more compact, etc.
Nothing particularly wrong with Sony, but the EOS R system is looking pretty strong for 2020, and it's only going to get better still.