Good point. But they share the same firmware unless I experiment with merlin and it is a bit out of my budget.
Nope, they use different firmware as they have different hardware components. Otherwise look for a used RT-AC68U or RT-AC87U on ebay, they can often be bought for < £100 - both will be significantly better than your current RT-AC1200G+ which is a budget Asus model.
Alternatively consider a Netgear R6300v2 such as this
. Its one of the best wifi 5 routers out there.
BT Business FTTP 330/50 -- Netgear RAX200