re: Worth upgrading 11 year old DSLR for Mirrorless (Nikon specifically)

The Nikon's are discounted as a precursor to the upcoming release of newer models. That said, the Z6-ii is a very capable full frame camera.

You currently have an APS(crop sensor) and the Z6-ii is full frame, so even with a FTZ adaptor your current lenses may not be usable. Going to full frame you will lose the 1.5 magnification of the APS camera, so a 70-200mm lens which would be equivalent to 105-300mm on an APS will only be a 70-200 on a full frame. Depending on the sport you're photographing, you may find that you will need a longer focal length lens (more money). That said there are image quality advantages that come with a full frame sensor.

The new mirrorless cameras will generally be smaller in size and weight, have more flexible auto focus options, with improved focus because the focus is read directly off the sensor. If you interested in video, you'll get much higher resolution and higher frame rates from a newer camera.

Biggest disadvantage of a mirrorless camera will be battery life (just buy more spares, if this is an issue) and the adjustment of that comes with the newer electronic view finders.