You have Nikon's cutting-edge 24MP 'upper-entry' DSLR in the D5600. You can't do better for this camera for landscape without significant investments in both the camera and lenses, and even then, it will be difficult to say you 'see' a tangible difference.

That is sage advice.

How 'serious' about wildlife? Are you up at all hours stalking birds and other animals, in all weather? Are you tracking birds in flight? You can blow a whole bunch of money on a few of Nikon's other cropped sensor bodies that are better at these specific tasks than your D5600. Even with a new body, you still need a lens appropriate for these all-weather, low-light shooting situations.

Your 18-400mm has more than enough focal length for wildlife, especially on the DX format body, although it's going to perform best in full light. Your D5600 has plenty of high ISO performance and frames per second to let you work outside of full sunlight.

If you are practicing wildlife with the 18-400 and disappointed with the results, you'll save your self a lot of money by simply posting some example JPEGs and storing the attached files and asking for help on your technique. An hour of coaching and 4 hours of practice are widely known to generate more and better results than another two (2) new lenses or even 30 more megapixels.

You have Nikon's cutting-edge 24MP 'upper-entry' DSLR in the D5600. You can't do better for this camera for landscape without significant investments in both the camera and lenses, and even then, it will be difficult to say you 'see' a tangible difference.

How 'serious' about wildlife? Are you up at all hours stalking birds and other animals, in all weather? Are you tracking birds in flight? You can blow a whole bunch of money on a few of Nikon's other cropped sensor bodies that are better at these specific tasks than your D5600. Even with a new body, you still need a lens appropriate for these all-weather, low-light shooting situations.

Your 18-400mm has more than enough focal length for wildlife, especially on the DX format body, although it's going to perform best in full light. Your D5600 has plenty of high ISO performance and frames per second to let you work outside of full sunlight.

If you are practicing wildlife with the 18-400 and disappointed with the results, you'll save your self a lot of money by simply posting some example JPEGs and storing the attached files and asking for help on your technique. An hour of coaching and 4 hours of practice are widely known to generate more and better results than another two (2) new lenses or even 30 more megapixels.

The zoom ratio of the 18-400mm lens is 22:1. This is a very wide range, and accomplishing it requires some important compromises along the way. Those are likely to be most apparent at the long end of the zoom range. Zooms with a ratio of 4:1 or 5:1 generally require fewer design compromises and tend to perform better, including being sharper at longer focal lengths. You may also find a 'faster' lens to provide you with some better results.

As for your camera, I agree that the D5600 is a generally competent machine. Its sensor is fine, but as has been mentioned, the viewfinder provides smaller and dimmer images than the improved viewfinders in more enhanced models provide. This can make it more difficult than necessary to
achieve best focus.

Depending on how you shoot and how you use your camera, other models can provide you with a much better user interface, including more directly accessible physical controls and requiring far less dependence on the menu system. The biggest difference is that the D7500 and D500 both provide two command dials, allowing direct and immediate control of both shutter speed and lens aperture. While this may not directly affect picture sharpness, it can definitely reduce distractions that draw your attention away from critical things that do.

Finally...good shooting technique is critical, especially when shooting with long lenses. Proper two-handed camera support (or a sturdy and effective tripod or monopod) is very important. So is acareful, controlled shutter release technique that doesn't jostle the camera and lens, even a little bit.

Improving some of these may involve spending some money. Others just require attention and practice. I suggest working on the procedural ones first, then consider whether your equipment is holding you back. At any rate, have fun as you work through to discover how to improve your images.

You have Nikon's cutting-edge 24MP 'upper-entry' DSLR in the D5600. You can't do better for this camera for landscape without significant investments in both the camera and lenses, and even then, it will be difficult to say you 'see' a tangible difference.

How 'serious' about wildlife? Are you up at all hours stalking birds and other animals, in all weather? Are you tracking birds in flight? You can blow a whole bunch of money on a few of Nikon's other cropped sensor bodies that are better at these specific tasks than your D5600. Even with a new body, you still need a lens appropriate for these all-weather, low-light shooting situations.

Your 18-400mm has more than enough focal length for wildlife, especially on the DX format body, although it's going to perform best in full light. Your D5600 has plenty of high ISO performance and frames per second to let you work outside of full sunlight.

If you are practicing wildlife with the 18-400 and disappointed with the results, you'll save your self a lot of money by simply posting some example JPEGs and storing the attached files and asking for help on your technique. An hour of coaching and 4 hours of practice are widely known to generate more and better results than another two (2) new lenses or even 30 more megapixels.

Ah. If you want sharper pictures you have to invest in:

1) super sturdy and lightweight tripod (not cheap);
2) remote release for your camera (wired or wireless);
3) a 4x4 vehicle (to get you and your camera to the right place to shoot);
4) an ephemeris (to get you ready at the right time to shoot);
5) a Benq or other pro-level photo-suitable computer monitor that's calibrated for Adobe or whatever you use;
6) you didn't mention - are you going to make bigger prints or exhibition-level prints?