Thank you. A quick look on the forums showed that for my ISP (xfinity) and that modem/router combo I cannot change the IPv6 settings beyond what is shown in my previous screen capture. ("Its because its set by the ISP. Its built into the firmware on these combo devices for IPV6 to be enabled. there's not a disable option unless the ISP requests that firmware option. Most don't.") Dang.
I will continue to see if I can find a way around it, but otherwise it looks like changing modem/router would be the only option for me to enable Pi-hole to work for all my devices without individually disabling IPv6 on each, correct?
edit: I saw another post on here that had the same issues but different router/modem.
I wonder if you have found any routers/modems that seem to work with xfinity/comcast and Pi-hole, or if there is a better route to simply assign the Pi-hole as the only DNS server. I tried this initially, since my router/modem can do so, but it did not block ads, so I went the DHCP route.