VLAN 1 and VLAN 2 are used at the driver level for GMAC0 and GMAC1 (see above, or target/linux/ipq40xx/patches-4.19/711-dts-ipq4019-add-ethernet-essedma-node.patch).
In my experience, VLAN 1 and VLAN 2 are best avoided in any "custom" configuration of the switch.
Bridging the same VLAN across eth0 and eth1 has some "interesting" challenges as well, seemingly related to one MAC for a given IP address being needed for connections on the "WAN" port and another on the "LAN" ports.