I live in a fairly large 5 bed detached and I swapped my BT Smart Hub for Deco M5 mesh wifi system a while back to try ensure strong wifi everywhere and to support three wifi cctv cameras installed outside which have struggled with signal issues.

I have x3 Deco M5s in the house, x2 downstairs and the 3rd upstairs with a 4th in the garage. I have FTTP Broadband with BT with max speed of 500mbps but deco speed performance hasn’t been near these speeds with the main deco tending to get 300mbps max speed when directly next to it and the second deco drops to under 200mbps and the third deco around 110mbps. Is this normal? It seems like big speed drops with the main deco getting no where near the 500 mb max speed I pay for even when directly connected via Ethernet. 

I was therefore considering removing the Deco M5s and going back to a router but a high spec one like the Archer AX6000. Will this improve the wifi speed throughout the house or will I actually see worse performance from this router at range e.g upstairs and for the cctv outside?

What's the performance on this router like for throughout speed and range? Would this router on its own or with a combination of powerline adapters likely give better performance than the Deco M5s?