Blurryeyed wrote:
Not a large enough budget.
But if I had to stick to that budget I would probably go with a Canon R5 and a RF 100-500 then sell an old EF lens to pay for an extender.
Both the lens and the camera are important, you can't underestimate the need for pixels to afford cropping, if the budget were a little larger I would go with the same camera and a used EF 400 f/2.8 IS II with both of the Canon extenders.