Self custody is a non negotiable starting point, so Phoenix fills that requirement while providing the best UX. I would probably avoid onboarding someone directly onto Lightning unless I know they will need to make small payments in the near future.
Self custody isn't "instantly perfect", it's literally the bare minimum that has to take place in order for someone to actually become a new Bitcoiner.
If I were to be a snob about it, I would insist they run a full node first.
Phoenix for someone entirely new...?