Geezus; there’s a lot going on in this post given all the other devices you’ve got going on. Here’s what I’m going to suggest: you’re going to find yourself wanting to get into LuCI & SSH sooner than later. Before doing so to do more advanced setups (eg: IoT VLANs &/or bridging VLANs), make sure you have a backup(s) to fall back on if things go awry. Here’s a HOW-TO:
You’re already running the latest release, OpenWrt 23.05. From there, feel free to consult the OpenWrt Wiki, maybe their forums, if you don’t find what you need in the GL docs.
… but I don’t see a reason why not to use the Guest subnet for IoT if you know you won’t be having any guests use it.
(You might want to map/diagram your current network vs your ideal network setup; see draw.io)