@David-TP Hi. Thank you so much for your reply. I'm struggling a bit with the diagram you posted so I have made one with our current set up.

We have one "Scenario 1" and eight "Scenario 2".

*The TP-link AV1000 models are either: TL-PA7027P or TL-PA7017

To clarify, the ethernet backhaul via electrics (powerline) comes out of the same electric phase to reduce interference. The current set up provides a single SSID with seamless roaming throughout the house. 

The question would be, what TP-Link DECO could replace all FRITZ!REPEATER 3000? We would ideally like the most affordable solution and we would like to keep the TP-Link AV1000 we already have.

Many thanks

  @calmgate 

Any Deco model with Ethernet port can use TP-Link powerline adapters.

I googled FRITZ!REPEATER 3000 and it says it is WiFi6 capable. If you want replacement to similar Deco, I suggest choosing from WiFi6 capable models.

The most budget-friendly among WiFi6 capable will be Deco X10, followed by Deco X20. Check their prices and availability, buy from these two models the one that is less expensive (but if at similar prices or on sale, X20 is preferred over X10)

There is just one issue with the diagram you provided, and I'd like to ask you to clarify that for me. According to Modem G.Fast MT992 specs I googled it is, indeed, just the modem. It has one Ethernet port and requires router. If there is router, what is its brand and model number?

The Fritz Repeaters by the looks of the model is an AX3000 unit the closest model in the Deco's is the X50 i have the pro model that has 2x 2.5GB ethernet ports and its an AX3000 WiFi6 unit if still available you can get up to 3 units and they support ethernet backhaul as well as wifi. I will be completely honest though they mesh better over wifi backhaul rather than powerline adapters i tried the TL-PA9020P Kit but the loss is not worth it for me i ended up going down the wifi backhaul and its been more rock solid unless you require more distance powerline kits are very last resort.