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I am mostly a Nikon shooter so not aware about the Canon EOS RP camera and its capabilities. However, one thing that I have observed and learned over the last couple of years shooting wildlife, birds and nature is the lens is more important than the camera itself. The better the lens, better results you will get even in low light.
So if your budget permits, go for prime lens like 600mm F4 or F2.8. Both these lenses are on the costlier side (on Nikon side they cost over 10 lakhs). If you want more versatility in your focal range then go for telephoto zoom like 150-600/200-500 or equivalent. These telephoto zooms are available from third party as well like sigma or tamron and cost much less as compared to prime lenses.
Another thing that you might want to consider is what is your most important subject i.e. do you mostly shoot birds in bushes which are shy, then the 600mm is not always enough. On the other hand if you are shooting big mammals and you can get in close proximity then probably 600mm would be too much.
So finally, it comes down to two important things while choosing the right lens
– What is your budget?
– What exactly you would be shooting?
I hope this helps.
P.S. My experience with 200-500mm lens even when paired with Nikon Z9, the images when shot wide-open and are at farther distance often turn out to be soft (again it might be because of my technique as well).