Hopefully a simple answer...

I've used three S4s since around 2020. Previously, in a compact 3 bed house; main Deco in the lounge (back of house), 2nd Deco in the dining room (front of house), and a 3rd Deco in an outbuilding (back garden). 

Deco 2 & 3 would connect to the main Deco, no issues, coverage was great (even the 3rd one which was at the end of a garden showing "Medium"). The Main Deco was pretty central to the other two.

We have since moved to a house with a much bigger floor plan and the Main Deco has to be in the Kitchen on the very front wall. Up until yesterday, a 2nd Deco was in the lounge connecting with "Medium" strength and the 3rd Deco was in the main bedroom connecting with "Medium" strength (labelled as V1 on the floorplan). I've only been getting about 96Mpbs when connected to the Lounge 2nd Deco so I've since moved the 3rd Deco to the dining room (V2 on the floorplan) which is now connected to the Main Deco with a "strong" connection. My understanding was that the 2nd Deco in the lounge would connect to the 3rd Deco in the dining room creating a "Strong" chain/mesh but this isn't the case. The 2nd Deco in the lounge is still connecting to the Main Deco in the kitchen with "Medium" strength and still giving me about 96Mbps where as the dining room one is full speed; previously, this didn't matter because my actual internet connection was only 72Mb (max we could get) but now we have full fibre at 300-330 so want to be able to acheive as close to that as possible in the whole house.

Main Deco: Plugged in to my router (modem mode) in Wireless Extender mode (can't get it to work as a router no matter what I try and no matter what my ISP does)

2nd Deco (Lounge): Ethernet port 1 plugged in to a NAS (Timemachine backups), Etherport 2 plugged in to a switch for: Xbox, TV, Hue Hub

3rd Deco (Dining Room): On it's own, nothing plugged in, just wireless.

I plan on getting another S4 to cover the upstairs of the house at some point, but I'd expect this to now connect to the Dining room Deco (at "Strong" strength) as it will be almost directly above it, but the behaviour of the setup at the moment suggests it will try to connect to the Main Deco at "Medium" strength.

So my question is:

• Should the 2nd Deco really be connecting to the 3rd Deco (to create a chain of "Strong" connections) or the Main Deco (creating a slow "medium" connection)?
• Will an additional S4 upstairs connect to the 3rd Deco or Main Deco?
• Is there a way to force these things to connect to each other or does it just "learn" what is the best way to do it?
• Should I just bin off the S4s and get something newer and more powerful?
• Should I get something newer/more powerful as the Main Deco and keep the S4s?

The house is a late 1960s UK house, brick-built with internal brick walls and the wiring is interesting, to say the least, so I'm not even sure I'd be able to fix this with powerline adapters like I used to in the 2010s.