I think your first approach (using string interpolation) was pretty close. If rather than putting lbfe_id into a list you instead use Token.as_list(lbfe_id) when passing to pls, it may work correctly. May also be able to do something similar when using the override.
I believe Use `ComplexList` classes for references to list attributes of resources · Issue #1610 · hashicorp/terraform-cdk · GitHub would make this a better experience.