Also, despite being Wifi 6, the Hub 5 won't have the coverage and range of the Deco. With the Deco you've got three access points, the hub is a singleton, and for the presumably majority of devices that run in 802.11ac or slower then they may well suffer significant degradation in performance if relying on the Hub 5 alone. Personally I'd put the hub in modem mode and rely on the Deco (I use a similar system myself with a Hub 3), and sit out the benefits of Wifi 6 until it is more common, and then, only if there's real domestic need I'd buy a Wifi 6 mesh.
You could of course mix and match - run the Hub 5 in router mode, run the Deco in access point mode, keep network names different and only give Wifi 6 devices the hub's credentials (as well as those for the Deco), everything else runs only on the Deco. Obviously you're taking a small chance on the competence of the Hub 5's router capabilities, and you'd be relying on the Deco and Hub 5 to play nicely in terms of channel choices, but if you've got the time and inclination that would be an interesting approach that might make best use of the Deco mesh, whilst still permitting Wifi 6 devices to try their stuff. With Wifi 6 devices having both networks credentials, you rely on them to auto-select as they see fit. If you do that, there's people here who would be intrigued by how it works in practice.
