It's probably much more cost effective to do what you suggest, and that's exactly what I do. Multiple SSIDs (one for the household, another for IOT stuff, another for work and another for guests) and control access via VLANs.
On my network we just have a guest network which denies LAN access to anything connected to it, but I'm wondering if there's a good reason to split IoT off entirely.
Things like my printer and wifi-connected grill live on a much more restrictive VLAN. (with some firewall rules to allow devices on the trusted network to still print to my printer's hard-coded IP address)
I used to do this but it became too much of a hassle.
I'd do it so that I could more easily prevent the IoT stuff from phoning home.
And to donate to OpenWRT, of course!