Hey All,
Thanks for all the posts above for the hints that they gave me. Additionally this post Xiaomi Router 3G V2 - VLAN with DSA helped me as well.
On the ERX-SFP, I have WAN connected to eth0 and br-lan connected to eth1. The rest of the ports are the ones I waned to play with. Here is what worked for me. Note, before the start of the script, eth2 and eth3 are not shown in bridge v. This is using the ramips-5.4+fixes branch from vDorst (thanks for all the excellent work).
ip link set eth2 up
ip link set eth3 up
ip link add name br-test102 type bridge
ip link set dev br-test102 type bridge vlan_filtering 1
ip link set dev br-test102 up
ip link set dev eth2 master br-test102
ip link set dev eth3 master br-test102
bridge v del dev eth2 vid 1
bridge v del dev eth3 vid 1
bridge v del dev br-test102 self vid 1
bridge v add dev eth2 vid 102
bridge v add dev eth3 vid 102
bridge v add dev br-test102 self vid 102
ip link add link br-test102 name vlan102 type vlan id 102
ip link set dev vlan102 up
ip add add 192.168.102.1/24 dev vlan102
On eth2 I have a device (192.168.102.2) which directly tags on the port. On eth3 I have a smart witch, which does the tagging and then forward untagged to another port which then goes to a device (192.168.102.3).
I can ssh from the ERX-SFP to either of the devices and from either of those devices to the other. I tried ping and ssh. Both work flawlessly.