Thanks for the replies, turns out the vlan stuff was easy, I was just looking at the wrong guide…
In /etc/config/network I changed the ifnames for the interface 'lan' to option ifname 'lan0.1 usb0' to have the “lan” on vlan1 of lan0. Also added a guest network interface using:

config interface 'guest'
        option ifname 'lan0.3'
        option proto 'static'
        option netmask '255.255.255.0'
        option hostname 'gw2guest'
        option ipaddr '192.168.10.0'

Then go into the luci firewall UI and set-up the guest interface with its own zone, etc.