If a person doesn't mind the weight, go with the EF 600mm F/4 II and an extender.
https://www.the-digital-picture.com/...mp=0&APIComp=0

And if Weight is an issue, go with the RF 600mm F/4.


https://www.the-digital-picture.com/...mp=0&APIComp=0

I think Canon called this stream lined engineering. Maybe they could slap another built in 2x on the 1200mm and have a 2400mm to sell for $40,000.