If you REALLY need full control over WiFi, you may need to switch to business-grade APs rather than using consumer network gear or a solution provided by your ISP. My APs allow me to create multiple SSIDs on the 2.4 and 5 GHz bands. I can create one WiFi network (same network name on the 2.4 and 5 GHz bands) with band steering enabled. If I need a device to connect to a specific band, I may not be able to do that through a band steering policy but I can create a secondary SSID with a different name, with no band steering.
I also have full control over channel selection and configuration, power levels, can fine tune how band steering works, my clients can roam seamlessly across APs, I can control how clients authenticate on a per-network basis, can assign SSIDs to specific VLANs, can (on my external router) implement different security polices and Internet access policies for each VLAN, etc. I have a more extensive configuration and management UI just for WiFi than the XB6 has for everything.
The downside to business-grade network gear is that it also lacks the conveniences and functions that most people want and expect; things like making it easy to add devices to the network using WPS, parental controls, etc. nor can you manage everything end-to-end using a friendly mobile app. And you will have to support all of that complexity on your own.
There are ways to do all of the things that you want to do. Business-grade network gear is probably overkill, but you can't get the level of control that you desire with a consumer-grade network gear... although, if installed properly, you will get a WiFi network that performs well and "just works" with a minimum of fuss to manage it, and full technical support when you need it.