Hi and welcome to the forum:
Quite a few changes have occurred in the 20 years since the 20D was released. DSLR cameras are now essentially being replaced by a whole new generation of cameras called Mirrorless Interchangeable Lens Cameras (MILCs). In many ways they share a lot in commons with the camera you are used to, but they no longer employ a mirror to reflect an image via a pentaprism to your viewfinder. Now, images are viewed and metered directly off the sensor and the images displayed on a tiny LCD screen Electronic View Finder (EVF), where the optical viewfinder was, there is also a large LCD screen at the back of the camera that can also show what you are shooting in in real time - it's called Live View.
Despite all these changes, you would find much that is familiar in the camera interface - many of the buttons and dials are similar (although there are more) and the menus, while expanded are treated the same way. With any new camera there is always a learning curve, and the longer the period between engagements the more there will be to learn. Still, I would bet you could pick up a MILC, recognize it as a sibling to your 20D and be able to operate most of the basic controls pretty quickly.
There are significant advantages to this. You can see what exposure will be achieved directly on the displays, and focusing is done using the sensor's far greater number of photosites, resulting in much more accurate focusing and exposure. This allows such features as auto eye focus, where the camera will look for and focus on an eye, and has the ability to track an eye or a face if it is moving - brilliant for wildlife. Most modern units can focus on people, animals and even find drivers in cars or on motor bikes - great for sports photography.
Sensors have improved massively. Sensors of 24MP are considered normal for both APS-C (like your 20D) or full-frame, but some can far exceed that, such as the R5 series at 45MP. Both ISO performance and dynamic range are also vastly upgraded, with an ability to record in 12800 ISO with minimal noise and good colour and tonal rendition. All of this is done at much higher speeds because cameras can use either a traditional mechanical shutter or full electronic shutter where the sensor simply turns on and off to record a new image - in most cases this is done line by line, but in a few cases they can all be refreshed at once, and this is called a Global Shutter.
Finally, cameras now offer In-Body Image Stabilization (IBIS) that can compensate for some camera movement in 5 axes and even work with lens Optical Image Stabilization (IS) to offer up to 9 stops of improvement, thus allowing steady shots in very dim light that look sharp.
Lenses have also improved significantly with improved stabilization, many more sophisticated coatings to improve refraction and reflection. Many lenses are now lighter, more compact and cheaper, yet offer top end results because of the marriage of pure optics with software that works with the camera to make corrections and produce high quality results without great cost. This is aa application of technology that was originally developed for cell phones and now allows much cheaper optics that perform at a high level.
With all of that said, we need to look at what you need. The reason I explained all of the above is because we would mostly recommend that you move to the MILC platform as this is where the latest developments are and the future lies.
To help us to help you, it would be critical to know the following:
A $amount for your budget - this for lenses and body
I understand your intended subjects, which cover a very wide range, which usually demands a mix of optics. What lenses do you currently have - please be as specific as possible as there are often multiple versions of one optical range.
Do you shoot in very dim light? What subjects are your highest priority?
What do you intend to produce? That is, will you create images for social media, digital display, small to medium prints, or extremely large prints?
Do you have any issues in carrying gear, also do you want lightness and compactness for travel or hiking?
Would you prefer a few lenses with a lot of focal range, or prime (single focal length) ones?
cheers, TREVOR
The mark of good photographer is less what they hold in their hand, it's more what they hold in their head;
"All the variety, all the charm, all the beauty of life is made up of light and shadow", Leo Tolstoy;
"Skill in photography is acquired by practice and not by purchase" Percy W. Harris
