I have the RF 100-500mm and it's been a fantastic lens for me on the R5. The image quality is excellent, and the reach is very useful for wildlife. I'd recommend this lens as it's extremely versatile - you can shoot wider scenics at 100mm and then zoom in to 500mm for distant subjects. It's also relatively affordable compared to the prime options.

Try the new released RF 200-800mm lens, if you can get one. 😎 

Have you looked at any of the EF lens options and using it with the adapter? The EF 100-400mm f/4.5-5.6L IS II USM is a fantastic lens and you can probably find one used for under $1500. Robust build, fast aperture, and great image quality.

I use the RF 800mm f/11 for bird photography and it's been great value for the price. Yes, the f/11 aperture is limiting, but I shoot mostly in good light. The compact size and light weight also make it very portable. Just something to consider if your primary use case is distant subjects in daylight.

If you want ultimate reach and flagship performance, the RF 800mm f/11 IS STM is very intriguing. It's insanely compact and lightweight for an 800mm prime. Just know you're locked at f/11.

I also love the new released RF 200-800mm lens.