There's no waiting to jump, I've had the 100-500mm and both TC's since summer. My wife enjoys shooting when the weather is good (currently with my old Nikon D500 rig) and on our last vacation she found the primes that I used for wildlife very limiting when there were other things to shoot, so I picked up the 100-400mm to go on my R6 for her to use moving forward. Unlike it's big brother the 100-400mm accepts both TC's with zero limitation on zoom range.
I wouldn't expect to use the 2x with it that often, but I wanted to suggest it as a more flexible budget option to the 800mm alone. I fully expect it to be sharper and quicker with the 1.4x, and it'll be a little friendlier at 140-560mm for my wife - and she can feel like she's getting a little closer than me, even if I've got more pixels on a bird.