You won't get concert hall like sound on a small budget but we can try and get the best return on your dollar. A search on the DZ680 shows that it is a one box solution for home theatre. These systems typically do not allow you to make any upgrades and the speakers are usually low quality. The speakers will have the biggest impact on sound quality. They appear to use a proprietary connector for the speakers so the speakers are designed to be used only with the DZX680 main unit. You can keep this setup if you have a room for casual TV viewing but it will not produce a theatre-like experience. I think it is best to start from scratch and build a nice 2 speaker setup for music. If you think you would like to add more speakers later for surround music or concert movies, try and build that into your plans. Selling the Sony system may help put more money towards something better.
The Denon AVR-2808 is an older unit with only 2 HDMI inputs and a lot of analogue inputs. I could not find the HDMI spec in the manual but you want at least HDMI 2.1 support or newer, otherwise your AVR could be obsolete from day one.
Getting a multi-channel AVR can be a good idea as they can be the same price as 2-channel equipment but have better video support and will allow the addition of more speakers later. It would help to know what country you are in and what other AVRs are available to you.
The Edifier speakers you mentioned are not designed to work with your Sony unit, or any other AVR like the Denon. They are designed as stand-alone powered speakers that you connect to a streaming device like a WiiM or to your phone via blue-tooth.
We don't see Eltax speaker in North America but a quick search reveals that they are by a French company. They are still budget speakers but general reviews seem to indicate that they offer decent value for the money, so there is a good chance that the Eltax will improve upon your current Sony speakers.
You will need to manage your expectations. Good quality audio comes at a price. If your main goal is quality 2-channel sound, 2 good speakers will do better than 5 cheap speakers. If having surround sounds is more important, you will have to make some compromises.
What is your source material? Music streaming services, TV broadcast, Youtube, your own physical media? Think of what connections you need to make on the AVR and if you need a TV screen attached.
Is your priority 2-channel music or surround music / movies? Can you start with 2 speakers and add more later or will this be a single long term purchase? What is your total budget for AVR and speakers?
