From your post, it appears that you may not know about the Advanced GUI (a.k.a. LUCI OpenWRT interface). Once you check out that GUI that is built into the Flint 2, and do a little research on how to configure additional LAN’s and VLAN’s in that OpenWRT GUI, then you’ll be able to setup what you’re describing.
Here’s a thread that has an associated video of configuring VLAN’s in the Advanced LUCI OpenWRT GUI.
Also, if the Flint 2’s Guest WLAN is not enabled, then the associated Guest LAN network will not be enabled and won’t work at all. So since you aren’t using the Guest WLAN on the Flint 2, then you could create another IP based LAN network in the Advanced LUCI OpenWRT GUI assigned to the Guest Firewall just like the Guest LAN with your VLAN 100 assigned to it.