I unterstand completely. In 2022, I went on a safari in Kenya. I was facing the same dilemma as you are right now. In the end, I decided to upgrade my 100-400mm lens with the RF 100-500mm and the 1.4 extender and I kept the EOS R. For most part I did fine and got great pics, but when I spotted a fast running leopard I only managed to get two sharp pics and basically 50 unusable ones. In 2022, prices for the R6 and R5 were still (also covid related) very high, so I opted the other route. The R7 didn't ship in time... my initial idea was to pair the R7 and the R. Oh well, still did good/ great pics.
For you, depending on your budget, I´d consider the following scenarios:
- get a R7 + 200-800mm and go with a two cameras. The R is great for landscapes and slowy moving animals like elephants and giraffes.
- get a used R6 + 200-800mm and take a two camera set-up as mentioned above. The R6 AF is not on par with mkii, but still lightyears better for wildlife than the R.
- sell the R, get the R6mkii
- sell the R, get the R6mkii, sell the Sigma and get your dream lens.
The price for the R6iii will be sky high for about 12-18 months. First cash backs/ discounts usually run at 250 $. Until it hits the price point of 2.200 $/€ it will be a while.
In Germany, the differences between a used EOS R and R6 on eBay are about ≈ 400 €, if you're lucky even just 300 €. Maybe you could sell the R, get the R6, be for happy for now and get the R6iii for Black Friday 2026/ 2027 and end up with a great two camera set-up. That´d be my way to go. If I ever go on a Safari again, I´d definitely bring a two camera setup.