@QuakerWildcat 

Just thinking out loud here.. but there is an option comes to mind

1. Take a note of your routers IP Address and Disable the DHCP on your current router.    Lets say its IP is 192.168.1.1
2. Disconnect everything else from network
3. Plug in the Deco to be master, to the switch.   When going through setup for WAN choose STATIC IP and set it for same range as router.. say 192.168.1.2     That should talk to the router and bring you online
4. Set up rest of deco devices via ethernet, they should all now be on a 192.168.68.x range provided by the only DHCP active (main deco).
5. Plug in / connect the other unplugged devices, again will pull a DHCP from Deco as router is no longer handing out DHCP.  

In theory that should work :)

@QuakerWildcat 

The bigger problem I see is your Deco mesh doesn't use Ethernet backhaul when Deco X60s run in Access Point mode. From your network diagram, they should. They absolutely should.

Three possibilities:

1. Your Deco mesh is actually running in Router mode. Double check settings in Deco app.

2. The Ethernet wiring is problematic.

3. Switches you are using are not compatible with Deco. See Q4, Q5 and Q7 from the following TP-Link FAQ: General questions about Ethernet Backhaul feature on your Deco

I would suggest fixing backhaul issues, before switching Deco mesh to Router mode.

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If you are not a network engineer and what @Philbert suggested sounds alien to you, consider buying additional single Deco X20 or X60, and adding it to your WiFi mesh. That would cost you few extra dollars, but it could be simpler solution.

Thank you @Alexandre. and @Philbert for reading my details and for the prompt, thoughtful responses. I was also puzzled by the backhaul issue and assumed it just doesn't work in my configuration where everything is going back through another router. Thanks for that link. One of my switches is, in fact, a D-Link brand. Swapping that out will be easy.

Thankfully everything works well at the moment so I'll chew on these suggestions and consider on a weekend soon whether to take on the more complicated or the more expensive approach.