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.
https://docs.gl-inet.com/router/en/4/user_guide/gl-mt6000/#first-time-setup
... 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)