I think most people here with handholdable telephotos do a variation of the same theme. This is my variation:
- Z9 + 500PF or 100-400
- An Arca-Swiss plate on the tripod foot, with Peak Design quick-release dongles
- The wide Peak Design strap, worn across the body
- The strap is cinched to rest at my hip when traveling so it doesn't sway, and a bit longer when shooting
- Hand strap on the Z9 to use while shooting, to reduce hand fatigue
If I need to shoot other subjects I also use either:
- My phone
- My X100V in a pocket (preferred) or on a short strap on my neck (for short trips)
- My Peak Design sling bag strapped tight to my back, with my XT4+18-55 or Z6+24-200
I'm not a large person, but I can carry the Z9+500PF for 4+ hours with no pain. Incidentally, I used to carry a Canon DSLR kit that was about the same size/weight as the Z9+800PF, and four hours was roughly the time when
that kit started making my shoulder sore.
Personally, I'd recommend using a cross-body strap for your kit, and making you can adjust it so it doesn't swing around when you walk. The movement of the lens is worse than the weight, in my experience.
Once you have the strap issue sorted, downsize your lens until you don't feel pain. If you have to move down to a 300PF and a 2x TC, that's just what you're going to have to do. Unless you're shooting for hire, I think trading comfort/health for image quality is worth it every time.
If you don't like the Peak Design strap, I know many people like Black Rapid, and I was very happy with my Op/Tech straps back in my DSLR days, too.