MT6000 custom build with LuCi and some optimization - kernel 6.6.x

I've got pbr working fine on my MT6000 using this firmware (be it with only 1 wg tunnel).

I suggest you post in the pbr thread, you are more likely to get the support you require there (they are very helpful and know their stuff about pbr):

policy-based-routing-pbr-package-discussion