Thanks for sharing this with us. I find the idea of self-configurable packages very cool.
Still, regarding the Symfony Flex Server, I think it would be better to make the composer plugin independent of this service.
You mention about to go opinionated again, but there is no reason why not to allow developers to use the plugin at their own will too, or not ?
Also, would it be possible to implement other configurators ?
thanks again for the post, I like it a lot.