Your camera choice will not require you to upgrade from LR6 to subscription. Before spending the money, it may be worth deciding specifically what you are looking for in a camera and then compare each camera's specifications. They are both incredibly capable cameras, you just need to decide if you need the extras with the D850. Good luck!
Unless you are shooting low light or fast moving subjects and would be better suited with the F2.8, this is a great lens, you will not regret it. Yes, the F4 is constant throughout the entire lens range.
I think you should forget that method and use the Focus Tune software that works with LensAlign. Peaking is OK, but it's subject to interpretation and a touch of variability. Using multiple shots and the Focus Tune software is far more precise.
That said...
Your D850 also has Auto AF Fine Tune and that can work well too - I've used it with my D850 for my 600 and 300 with and without TCs. I'v never been happier with my AF Fine tune results. However, it has to be done correctly. Here's how I do it:
Finding that perfect backpack isnt easy. I use a streetwalker for lighter loads and a mindshift, when I'm hitting the trail. You may want to check them out on B&H. Mindshift has a 26l and 36l.
Also, don't forget the bear spray and make sure you bring someone else with you, that you know you can outrun!
Shooting RAW provides more available information in the file and it gives the photographer more control over processing the photograph, which helps the photographer get their desired results. I personally only shoot RAW.