andi-: That should work. Looking at his module for it... you may want to also add "ppp_generic" to boot.kernelModules. And with PPPoE, it's generally a good idea to do MSS clamping.
Shados: that part he can tackle. It was more like "What's the laziest way to get the pppoe interface." He is migrating a Debian devices to NixOS and there you have first class pppoe support in ifupdown.