Just get the EF 300 2.8 L II, it's quite lightweight for what it is and it's at least on par with Sony's new 300mm prime. It also works perfectly with the 1.4 and 2x converters, without sacrificing too much image quality. I got mine used in mint condition for around 3.500€ (retail price in Europe was 6.599€).
If Canon would release a RF version of this lens, it will for sure be minium 6000€ at least.
It's the sharpest lens I ever used and as a bonus, it comes with some decent "3D pop".
Back to topic: If Canon really releases a relatively inexpensive 500 f/5.6 DO, which also would be leightweight and compact it will be my "shut up and take my money" - moment. In the last couple of months, I've found myself more than often, looking jealously over to Nikon's wildlife lenses.