Gateway metric virtual dynamic interface

Hey community,

I'm sucessfully using openWrt with my quite specific PPPoE DS-LITE wan connection. As a precursor for a mwan3 failover solution I wanted to assign gateway metric values to my wan IPv4 and IPv6 interfaces. But as those are virtual dynamic interfaces which are created upon successfull PPPoE connect, I'm unsure how to set/config the respective values. Setting them on the parent interface (wan) where PPPoE is configured did not pass them on to the virtual interfaces.

Any ideas?