@Bakewell 

Hi, Thanks for the message.

Archer NX200 is quite similar to your current 4G router-Archer MR600, especially in the web management interface except NX200 supports 5G NR. Archer NX200 supports EasyMesh but its current firmware doesn't support Ethernet wired Backhaul yet. So currently, the EasyMesh with NX200 could only be established wirelessly with EasyMesh range extenders.
https://www.tp-link.com/en/easymesh/

Deco X50-5G is a little bit different and it is mainly controlled by Deco APP. Deco X50-5G would be a better choice for a Whole Home Mesh system with Ethernet wired Backhaul

Deco App User Guide (for 4G/5G Decos)_Deco 3.1 App

So you might need to decide whether you refer web UI management or APP control first, then check whether there are some essential software features you wish to retain for us to conduct a more detailed comparison.

Thank you very much and best regards.