You'll never have enough reach.
At some point you need to accept that reality and simply come up with the optimum solutions for what and where you make photographs.
The 100-400 is a tough call. It covers a very nice range, has excellent close focus, and provides lots of flexibility. But 400mm is not very long and the 600mm PF is probably the next level to complement the 100-400.
I have the 800mm PF, and it is too long for many situations. It's ideal for birds - especially small birds. It's hard to have much environmental context with large mammals.
My main kit is the 70-200, 400mm f/4.5, and 800mm PF. I've chosen to have gaps and clear choices. For long lens choice, it's often a choice between mammals and birds. Large wading birds fall into the mammal category and usually can be photographed with the 400mm lens, but small birds or jumpy subjects always require something longer.
The 180-600 is probably your answer. Depending on what you are shooting, you'll either take the 100-400 or the 180-600. The price is reasonable and optics are good. The 600mm PF is likely the preferred choice, but as you point out, it is much more expensive. The 180-600 is light enough that you can handhold it or use it with a monopod/tripod. It may not be your choice for travel, for butterflies and dragonflies, or for modest hiking, but it covers everything else pretty well. You could also consider a 1.4 TC for occasional use, but you will have a slight drop in sharpness.