You are right, the situation is better that what I thought, the commercial smbstax SDK might not be absolutely required.

They even have some BSP stuff freely available which is really positive sign. https://github.com/microchip-ung/bsp-doc

This would be such an enticing project to jump into, there are just enough bits to make it seem doable, even if the realist in me knows its not really feasible.

Intrigued by that, I went looking. First commit of support was 2019, for Linux5.2. Yet, 5 years later, there is not one example (that i could find) of OpenWrt running on these Chips. If the platform/FW/BSP package were any good, I'd expect there to be chat about it in the forums, and/or attempts to port to such devices from SMBStaX (The Microchip BSP Linux dist).