Sounds reasonable. I found the system and menu easy to learn and memorise after some weeks. I am still learning the nitty-gritty after some months. Some tutorials and testing gets one a long way.
The 100-400 II is a great lens, and I did not intend to push you towards the 300. In many cases, the zoom is better suited, especially with its versatility and when uncertain. It is light and small for what it is, but it is big if you never handled such a lens. You will get used to it.
I recommend connecting a plate or anchor point to the lens foot and attaching a strap to it (I use a Peak Design Slide, but you can use whatever can connect to the plate). This will take some pressure off the camera mount and you, as it is better balanced, as well as being easy to just swing up. That goes for any lens which is much heavier than the camera.
Have fun, feel free to ask questions, and I am looking forward to see your images.