Before you read anything below, understand that these are just my opinions, and food for thought on your situation.
When you begin looking for a new camera or camera equipment, really consider what spending a good deal of money on a new camera will bring to you, or will allow you to do/make. Keep in mind, a new camera won’t necessarily make your photos better. I’ve scrolled through your numerous spotting topics, and you’ve taken some really great photos. If I remember correctly, FedEx even reached out to you about one of them - which I think speaks for itself.
Just remember, a “good” camera is subjective. It all boils down to the person behind the device, because a “good” camera won’t automatically create great images, it’s still just as easy to take a poor photo as well.
Now, I can’t necessarily speak for the three you’ve mentioned above as I’m not a plane spotter, nor have I ever talked about photography with them, but as a keen hobbyist/advanced amateur in photography (I mainly photograph landscapes and wildlife), I’ve come to realize that my understanding of the technicalities in photography have helped me more than the camera I own. I’m sure they might agree with me that while yes the camera itself helps them take great images, it is the knowledge of the person behind the camera that makes the biggest difference.
Now you’ve probably heard the saying “the best camera is the one you have with you” and while that’s mostly true, I think the best camera is the one you master. Once you’ve mastered that camera and your skills exceed what the camera can offer, then it will be time to “upgrade” or invest in new gear or a new camera.
If you truly want that new camera/setup, which I know can be really enticing, then I voted in the polls above to:
a) choose the individual parts
To part a) Specifically choosing what you know you want will be better than being stuck with something you don’t really want just because it came in a bundle. (That is unless the bundle has the individual things you want)
To part b) I personally use Sony’s full-frame mirrorless line of camera and would highly recommend Sony’s imaging devices to anyone. Sony’s full-frame lineup is rather expensive, and lenses for their full frame system aren’t cheap either, however if you’re interested in venturing into Sony’s realm, their APS-C mirrorless cameras are just as good, if not better especially in terms of price (they’re considerably cheaper than full-frame), and have some of the best autofocus, and continuous autofocus systems on the market.
Best of luck with your decision, and I look forward to seeing many more spotting topics from you in the future!