First off, congrats on your upcoming trip. My wife and I are in the early stages of planning an adventure cruise up there in the next few years. I hope to see some of your pics.
I own/have owned the following: R7, 70D, 6D, Sigma 150-500, Sigma 500 4.5 EX, Canon 17-40. Obviously I don't have the same stuff as you, but I do own the R7, and the other pieces are similar enough for comparison.
IMO, having more reach is a frame of mind. 600mm on a 7D2 is pretty darned good. You will always dream of more reach than you have, so of course it's nice to think of. I do think the R7 would improve the the "reach" slightly with more pixel density and amazing autofocus. But I don't think the difference will be as great as you want. You'd probably get better reach by upgrading to a 600 F4 lens, but I understand the cost and luggage hassle, so I only say that to make a point, not an actual suggestion. The 150-600 is already a great travel lens. I took my 150-500 to Alaska years ago and shot with a 1D (ver 1) with only 4.1 mp. The pics I got are still some of my favorite wildlife shots of the past 15 years. And that's an equivalent 650mm vs your current 960mm. Beyond 1000mm you're already getting into atmospheric distortion territory, especially with larger animals like seals and polar bears. I bet the 7D2 and 150-600 will do you just fine on the long end.
This leads to the next issue. I've read on forums repeatedly that the current Sigma and Tamron superzooms can struggle with autofocusing on the RF mount. My 150-500 and 500 4.5 both had HSM and definitely didn't play well with the RF autofocusing system. Even though they were older lenses, my experience matches what I've read about pulsing issues with the R5, R6 and R7 on the newer 150-600 glass. That's why I ultimately sold them and bought an old 400 DO, because even the older Canon glass behaves predictably with the RF mount.
If I was in your situation, looking to extend my kit with a second body, I would think the 5D4 is the way to go. I have the little sibling 6D and it's awesome. You'll probably use your 7D2/150-600 most of the time. Slap the 24-105 on a FF for a perfect dual camera kit that covers all your bases. Then flip the lenses to modify the focal length range if you need to, and literally eliminate the gap between 105mm and 240mm (150mm x 1.6). I bet the 150-600 would be amazing on a FF. That way you get some nice flexibility for different shooting situations.
Just my 2 cents.