FOr what little it's worth, I've had good success with the 100-400 L II (whatever f/stop it is) on my R5, and, though I never owned a full frame DSLR to compare it to, I'm happy with it.
I was in the same boat as you actually, not wanting to get a 20MP camera. Other than that I'd probably have liked the R6 better; dual SD card slots and another dial instead of that little LCD display. Instead I have a CFE card slot I'll probably never use.
From what others have said, I see no point to going to a 5D IV instead unless your budget is tight.
FOr what little it's worth, I've had good success with the 100-400 L II (whatever f/stop it is) on my R5, and, though I never owned a full frame DSLR to compare it to, I'm happy with it.
I was in the same boat as you actually, not wanting to get a 20MP camera. Other than that I'd probably have liked the R6 better; dual SD card slots and another dial instead of that little LCD display. Instead I have a CFE card slot I'll probably never use.
From what others have said, I see no point to going to a 5D IV instead unless your budget is tight.
Ok, so to conclude, I have placed my order. My budget is a bit tight so I have ordered the R5 Body, The RF to EF adapter, a spare battery and a Sandisk 512GB Cfextreme card. I will use the 24-105 from the 5D3 for now until I buy the 24-105 for the R5. I hope to get everything by Friday.