@Handpol 

Let me see if I read your post correctly. You plan to buy 3-units mesh, and you will wire all three units with Ethernet cable.

If this is the case, it is easy to give recommendation:

You want future proof your home mesh system with WiFi6 - buy X20

You want budget friendly mesh system, but can live without WiFi6 - buy M4

Internet speeds of up to 500Mbps can be achievable on M4, if it is wired, wireless device uses 5GHz, located at optimal distance of 1-4 meters from M4 and has unobstructed view of M4.This is close to peak you could get from M4, you might get better from X20 if wireless device supports WiFi6.

Note that you will need way less than 500Mbps for simultaneous 4K streaming on multiple devices, and for Zoom, Skype, VoIP. 

The only suggestion I would make, regardless of if it is X20 or M4 you go with, is to run Deco mesh in Access Point mode in case you are only buying Deco for better WiFi signal.

By default, Deco runs in router mode. I would recommend switching Deco system to Access Point mode after you configured Main Deco but before you started to add Satellite Decos to your mesh.

See TP-Link knowledge base article How to set up your Deco in Access Point mode