@AristoMoggie 

Yes, your approach would work! Using powerline adapters to connect your Deco units is actually a valid way to establish Ethernet backhaul. The powerline adapters essentially create an Ethernet connection through your home's electrical wiring, and from the Deco's perspective, this functions as a wired Ethernet backhaul connection.

How it works:
Ethernet Backhaul is a feature that allows different Deco units to transmit data via Ethernet (wire) instead of Wi-Fi (wireless), increasing throughput and optimizing network stability. Once Ethernet backhaul is established, Wi-Fi backhaul will be disconnected automatically, and the Deco APP will display the backhaul status as "Ethernet" as well.

When you connect powerline adapters to your Deco units, they create an Ethernet bridge through your electrical wiring. This will allow your BE65 units (and BE25 units if you can add adapters there too) to communicate via this wired connection, freeing up the WiFi bands and significantly improving overall mesh performance.

Important recommendations:
• Make sure all Deco units are connected to the same power grid
• Use wall sockets rather than extension boards or power strips, as they might block powerline signals
• Surge protectors and air switches might also interfere with powerline communication
• Ensure your powerline adapters support gigabit speeds to take full advantage of your BE65/BE25 capabilities

This solution should definitely help with your signal strength issues and reduce the dead spots you're experiencing!

  @AristoMoggie 

Just be aware that my testing shows that most of the time you will only get max 50Mb/s out of powerline.

So do some internet speedtests with your setup the way it is, if you are getting more than 50Mb/s now then you are not going to see any improvement with a powerline backhaul.

If you are getting like 8-10Mb/s with the Wi-Fi mesh then yes, powerline will help. But noth as much as running ethernet between the Deco's if you can.

BTW, whats is your incoming internet speed from your provider? If your incoming internet speed is less than 100Mb/s then you probably won't notice the powerline speed drop.

Lots of things to consider. Of course you could buy from somewhere they are easily returned if they don't satisfy your requirements.

  @AristoMoggie 

Hi, welcome to the community.

It is indeed feasible to use PLC adapters to establish a wired connection between a satellite Deco and the main Deco via a Powerline connection. However, under this network topology, the speed that the satellite Deco obtains depends on the quality of the connection between the PLC adapters. If the Powerline rate between the adapters is low, the actual speed obtained by the satellite Deco may not be better than if it were connected wirelessly to the main Deco/other Deco nodes. However, if a specific area suffers from very poor (or virtually non-existent) Wi-Fi coverage and you intend to place a satellite Deco there, connecting it to the main Deco via a Powerline connection may be a better choice. 

You can refer to the articles below to see how to improve the Powerline rate between the powerline adapters and to better understand the relationship between the Powerline rate and actual network speeds.
What can I do if my powerline rate is very low?
Explanation about the Powerline Rate and Actual speed of the Powerline Adapters
 

P.S. If you are utilizing only Deco units (without PLC adapters), you can follow the link below to place your Deco for better performance.
Deco Placement Guide: Where to Place Your Deco Mesh Wi-Fi Units

Best Regards