Which pair of big lenses for wildlife from a hide and on hikes...

I am lucky enough to own both a Z70-200/2.8, a 500PF and the 800PF lens as well as a Z7 and Z9. The 500 and 800 have a slow aperture, the 70-200 does not, but it has low reach. My best pictures of birds and mammals are typically taken from hides, but being in northern Europe there often isn't a lot of light, for which reason I consider buying faster glass. I am considering a F300/2.8 (which can be had second hand for 2-3000 Euro), a Z400/4.5, a Z400/2.8TC or a Z600/4TC. One or more of the other lenses will have to go in that case. Except for the Z400/4.5 portability would be limited, but for work in a hide that is not a big problem. On the other hand, I do take the Z9 with a telephoto lens on hikes. The 400/2.8 is likely the best for mammals and large birds, where the 600 let alone the 800 may be too long. I would like to narrow it down to two lenses:

Two appealing, although costly, alternatives would be
- 400/2.8TC + 800/6.3TC
- 400/4.5 + 600/4 TC

Other suggestions?

I used a Nikon 400/2.8 AI-S for fifteen years. I currently use a Canon EF system including 300/2.8, 500/4.5 L, and manual focus 800/5.6. I primarily use mine for sports but living in a wilderness area I have managed a few unintentional, but excellent, wildlife mammal images. I say unintentional because I don't have the best AF system, I don't use blinds, and I don't yet have a gimbal. I have just started getting into birds. Perched is working well, without the above equipment BIF is proving extremely challenging.

Some of your lens nomenclature is confusing me, however here are my thoughts.

The 500/5.6 PF is only 1/2 stop slower than 400/4.5, so no real advantage for the shorter lens. Aren't the two 800mm lenses not the same f6.3? The only faster 800mm lens is f5.6, so again not worth switching.

I chose the 500/4.5 because it is handholdable compared to a 600/4. If you have learned to deal with the size and weight and a gimbal works for you, I would consider one of the 600/4 lenses. Not that you would need to add a 1.4x, having an 800 PF, it is a viable option. I know I would also be open to using a 600/4 with a 2x as well. This makes the 600/4 a very flexible lens.

I usually found the 400/2.8 too short for motorsports, and I would find it short for birds/mammals as well. If buying a heavy lens I'd prefer the 600/4.

If you find the 500 too long on a regular basis then certainly the Z 400/4.5 or even F 300/4 are good alternatives.

The 400/4.5 + 600/4 TC should be a great combo.

I own the 400mm TC and the 800mm PF. The 800 is (for me) easy to hand hold when walking to find birds. The 400 TC is hand-holdable (if that is a word). For me, the 600mm TC is not really hand-holdable and is more difficult to travel by air due to its size. With the Z external 1.4TC, the 400mm TC becomes a 560mm f/4 and 784mm f/5.6 lens - effectively a 600/800 lens, and of course it’s a 400/2.8 w/o a TC in use. While the 400TC may not be as sharp as the 600mm TC at 600mm, it is very very close and produces very high quality images.

I owned a F-mount 300mm f/2.8 in the past. Sharp lens, but rather short focal length and it is fairly heavy.

My use case differs somewhat from yours, in that I do not use hides often and I use a gimbal + tripod less than I do a monopod or hand-hold. For me, the 800 PF and 400 TC plus external Z TC when needed works well. I have taken the 400 TC with 1.4 external TC on hikes of up to about 5 miles with no issues. I use a cross body strap with a QD connector (Magpul) plus a Arca-Swiss lens foot (Kirk) with a QD socket to carry either the 400 or 800 mounted on a Z9.