@Agni07
Hi it depends what you are after tbh,
As said above there are many different options out there currently. Mesh units or routers you can have as access points, repeaters or actual access points.
From my very basic understanding from what I've seen most brand mesh units generally tell you what area they can cover normally in Sqm the higher the area coverage the stronger the signal i would say in using common sense to be true .
From what I have read online when companies test the devices area coverage open spaces to give the maximum area coverage so when it comes into individuals properties their mileage may differ greatly as each House is built differently with different materials and densities Which affect wifi signals greatly. The thicker and denser the material the weaker the signal will be amd some electrical devices can interfere with wifi signals.
From say plasterboard to brick to solid concrete ect, steel beams are also a wifi killer aswell amd how many walls the signal has to pass through to get to the device.
Im a believer of if its staying where it is amd it can be hard wired than thats how it will be connected and wifi is fpr the truley mobile devices that you cant get hard wired.
I cant say that if you upgrade to a newer router/s or mesh system that you'll get better WiFi signal coverage as you have said the walls are 5 inches thick and that in itself is a wifi killer especially on 5ghz as it maybe faster but doesn't travel or penatrate walls as far.
I personally have 3 ax72 routers,
1 main and 2 satellites in AP mode.
This gives my house the complete full wifi6 coverage plus the added benefits of 4 extra ethernet ports per unit if needed for hard wiring devices.
Where your router b is I'm going to believe that there is a ethernet there for router b to work.
Are you utilising all the ports on router a +b?
I ask this as you have 2 options
You can either buy a mesh setup granted you do get more flexibility with them as you can put them anywhere within reason as you can do a wireless mesh system but that comes at a cost as the signal max bandwidth they receive is the max they can output.
(I.e
if you have a gb isp connection and because of your said walls at your property you can only receive say 300mb to that mesh device from the main node it can only output upto that 300mb max, if they can be hardwired then thats a different story as they'll be able to output upto the gb speed. These numbers are theoretical on the assumption that that the mesh devices can output those and you have those speeds from your isp.)
and then add say a small unmanaged switch to give you the extra ethernet ports you will need for your wired devices but you will then need a extra power socket to power it, as I believe not all mesh units have more than 2 ethernet ports you can use where as if you use all the ethernet ports on the router you can buy an upgraded router and still have the 4 spare ethernet ports needed and then if needed later on you can then add a small switch to give you the extra ports you may need.
It all comes down to what you need, the features and the price obv has a massive factor aswell,
I managed to buy all 3 of my ax72 routers for under £300. So comparing that to some of the mesh systems that are out there it made no sense for me to go down the mesh unit road as by doing the way I have in my house worked out better for me in terms of coverage, the extra ethernet ports, the looks of the units that's going to be in the property, your actual wifi speeds you pay for and your expeteded wifi you hope to get to your devices.
Some routers can't give you full gb wifi where some can the same as some devices can't do gb speeds where some can.
Bit long winded but it's not a very simple question to answer as there is a multitude of different factors that need to be looked at.
Hopefully this may help you in deciding what you decide to do