But I'd love to be able to separate IoT devices across multiple networks, so I could have one for smart switches, one for TV/media players, etc.
Of course, a lot of consumer-grade hardware won't do 802.1x so you end up stuck with needing a bunch of SSIDs (and wasting air-time on beacons).
Yes, that's why I want more SSID's. My internal network does use 802.1x, but as you said, few devices outside of laptops support it.
Not many kernel can separate traffic within a single SSID (even if you did use VLAN, tcpdump on a malicious IoT can still view the traffic.
Better to have four to seven SSID, each mapped to a subnet. Make one subnet/SSID for encrypted MAC with laptops.
Cable TV, Smart TV, power-line LAN adapters, smart lightbulbs and webcams should go on separate SSID/subnet.
You will not have to trust the potentially outdated wifi firmware, that is quite likely vulnerable to all the latest holes in wifi security.
It's worth noting though that more than 3 SSIDs brings with it radio overhead which can degrade performance of your wireless network.
I've also thought about just setting up multiple nodes, one set for secure devices, one for insecure devices.
Cisco "only" supports 16 SSID's, which is probably more than I'd even need.