Yes that's the way I had it set up. WiFi to WiFi from Smart Hub to disc, then connecting my device via ethernet to the disc. I got around 440mbps, and that was with one thick and thin wall in between me and the router, but I am on a 500mbps package.
You are going to be hard pressed to get 900mbps over WiFi, you might hit the 700s, but you have to be very close to the router to achieve those speeds.
1.) The next best Ethernet connected alternative is Powerline, which uses your house's electricity to deliver data, but the rated speeds on those are to be treated like a theoretical WiFi speed, which is to halve it and then that should be the maximum speed you can get from them. Powerline isn't all too reliable though for everybody and I can't use them at all in my place. But you may get it working perfectly for you.
2.) If you are set on WiFi: The next best thing is to get a WiFi 6 router, one which has support for DFS channels and then connect your WiFi devices this way, which should be able to hit higher speeds than your Smart Hub can do because of the higher power transmission and clearer range of DFS section of the band, but you have to make sure that you do not live near an airport or near any radar stations as DFS channels share the same frequencies as radar. You also have to make sure that all your 5ghz WiFi devices can support DFS channels as not all do.
With my WiFi 6 router and my WiFi 6 laptop, I can hit around 300-430mbps using the DFS channels and 160mhz bandwidth, and that is through one thick wall and 2 thinner walls. On non DFS channels on 80mhz bandwidth that drops to 100-120. But, again, even with this setup you are going to be hard pressed to achieve 900mbps over WiFi.
WiFi 7 is set to improve that with the new 6ghz band, as the extension WiFi 6e did before, and with this it SHOULD be possible to get even 1gbs over WiFi, and it doesn't need to adhere to any rules of radar interference, but the downside is that 6ghz has an even shorter range than 5ghz. You have to literally be in the same room as the router or have line of sight to it, over slightly longer distances, to make it work.
3.) The last of my suggestions, but still a good alternative, is to get an expensive mesh system that has tri band support, and then plug your Xbox in to one of these mesh units. They offer up faster speeds than BTs discs, and it will help to spread the speeds of maybe 400-600mbps around your home, but you're still dealing with the unstable medium of WiFi.
I will tell you that even 300-400mbps is a very good WiFi speed for a console, your kids won't have to wait too long for their games to download and for multiplayer gaming it is far more than enough to game with even 16 people in a lobby without breaking a sweat. It's around the speed I have to my PS5 (420-480mbps) and I'm very happy with it.
The 900mbps product isn't really intended for 1 high speed device, it's meant more for bigger households that then can use all their devices at the same time without the connection breaking a sweat. It's why BT doesn't provide a super high powered router or disc with it.
I think you can order more discs by contacting BT and then you have to give them a reason as to why you need more? But I am not sure. You can also get them second hand on Ebay too.
One thing to look forward to in the future: By the time games have grown to a truly humongous size, that faster connections than 900mbps are a necessity, then WiFi itself should already be a few versions ahead with a lot of leaps and bounds faster than today's current speeds.
Sorry for the long post, just trying to cover everything. If I have gotten carried away with myself, just please tell me and I'll try to explain it in more detail as best of my ability, and the other folks on here know a lot about these things.
