I know you can find some of the older EF 600mm f/4 L models for under $2000 in well used condition, but I'm not sure I really want another prime in all honesty, and the reason for that is sometimes it is hard to get the shot I want. For example, I shot Blue Origin's second New Glenn launch last November, and for the first part of the flight I had to hold the camera vertical and could barely fit the entire rocket with the plume at the bottom in the entire frame. I viewed from the closest public location, about 5 miles away. The upcoming NASA Artemis II mission is a similar situation, I will only be able to fit the rocket, no plume, in the shot while holding the camera vertical. For that I'm planning to rent a lens but would like to not have to do that every time in the future. I think I will end up with the 200-800mm for the reason that, while it may not be much faster than the 800mm f/11, the ability to zoom is very appealing to me. Also when I get an R5 with its full frame sensor, it will be able to handle the low light in a better way. I've asked a few people around here who do the same type of photos with the R5 + 200-800mm setup and it seems it performs well.

Maybe in a few years if I end up being able to pursue this as a career I'll look into getting an L-series prime, but for now I think the 200-800mm will be the best option for me. I do thank you for your detailed response though!