I was actually thinking the same 2.4 GHz IOT SSID on both AP's, and the same 5 GHz IOT SSID on both AP's, but your approach is even simpler. With your approach, you will not have to use mac address filtering to force clients to particular AP's.
When you are ready for VLANs, This post and and the breadcrumbs it leads to may be a good start for you: https://forum.openwrt.org/t/what-switch-and-ap-running-with-latest-openwrt-would-you-recommend-me-for-my-new-iot-network/149774/19?u=eginnc.