Tried again today, and now it does show lower tiers. For 5-year, from my current Fast (600/35, $89), I can get 300 for $-19, 500 for $-4, 1G for $+11, and 2G for $+26.
I am in a mid-split area, but am using an SB8200 and Asus RT-AC68U. Data cap is not an issue as I have yet to hit even 400GB.
If I go for the 500, or even 300, what gateway would Comcast send me? I have a feeling an XB7 would be the first choice for them? From what I can see, XB8 support Wifi6E, and XB7 is just normal WiFi6? Yes, I know I could always use bridge mode, too.