This sounds to me like a different config needed in OpenWrt, ie. from "bridge-VLAN-filtering" to DSA slave interface per VLAN.
If I understand this somewhat right, is the problem specific to OpenWrt that there aren't yet tooling to make that happen? Or have I understood this badly wrong.
In past I have configured a OpenWrt system or two to work without its default setup to bridge all interfaces except wan. In these devices, one was multiport x86 geos, getting rid of the default bridge setup improved the system performance significantly. My remarks were that cpu was totally overloaded by handling physical interface irqs while in default bridge setup and loading the system to its max. The system performance fell far away from what it was reported to perform (wirespeed in duplex 100M) and the nominal per interface capacity.