I have used pfsense and more recently opnsense. I just moved to openwrt for a more basic approach for now. I too needed to make VLANs which I have managed to do and have guest, IoT etc all working. The problem with the glinet gui is that it's mainly hardcoded. We get Lan and clients and that's about it in the dashboard, unlike opnsense where you get everything in the GUI.
Any interface you create in opnsense will appear in the GUI of opnsense and that's because it's the main overlay / frontend.
With glinet products the actual main interface is the openwrt gui which is called "Luci" and that's where you are going to see interfaces etc that are not the default.
I wish glinet was a bit more flexible. For example, it would be great if we could add an interface (guest Lan, IoT Lan) and it just appears in the main GUI but for now it's all under the hood.