I think the difference between your setup and mine is on the br0.7 config. I only have my default br0. No VLANs enabled. Attempting to give my instance of pihole a different IP worked to access the interface, but I think I probably need to configure it via VLANs for it to work.
In the meantime, I just put pihole on an old model B. Would rather have it in unraid.
What's your config for networking?