WAN failover is described here:
https://docs.netgate.com/pfsense/en/latest/book/multiwan/index.html
In addition to using policy routing you can now also set the system default gateway to a failover group which will also route traffic from the firewall itself.
You can, and probably should, have both WANs set as DHCP. It may be showing as unknown if the service is not started or there is a subnet conflict perhaps. More likely the latter.
You IoT/Guest AP should be in a different subnet to the LAN. That interface should be separate. The pfSense interface IP would commonly be .1 and the AP IP should be either statically set outside the dhcp range on that interface or set via a static dhcp mapping so you always know where to access it.
IPv6 should probably be set to track PPPoE unless you have a static IPv6 address.
To be able to have guest and regular users connected to either AP you want to set them up with multiple SSIDs on different VLANs and separate the traffic that way. Both APs should connect back to pfSense on the same port carrying all VLANs.
VLANs have nothing to do with connecting between subnets, each VLAN would be a separate subnet. Traffic from the IoT/Guest subnet will be able to reach resources on LAN as long as there are firewall rules allowing it. Conversely you should have firewall rules blocking access to the LAN for most guest clients.
Multicast/broadcast services, like Chromecast, are a different matter. They are not intended to be used across subnets and additional measures are required (igmpproxy/pimd).
Anything that should be in the same subnet should be on the switch. Interfaces on the SG-5100 are not switch ports and though they can be bridged to act like switch ports moving traffic between them requires valuable CPU cycles they could be used elsewhere.
The lag introduced going through a switch is negligible.
IP cameras would normally be considered an IoT device. Commonly found with known firmware vulnerabilities and no updates from the manufacturer. That would denote they are put on a separate subnet with very limited access to anything else. However they also generally generate a lot of traffic which will all have to be routed by pfSense if they are separated from the NAS like that. The decision is yours!
You can apply a basic priority based shaper to prioritise traffic from the xbox IP. It will need a static dhcp mapping to do so. However I would not do that unless you are actually seeing latency issues. Adding traffic shaping often introduces more problems that is solves.
Steve