You are correct. You can open a SE 401(k), but any contributions must be made with business income, not your W-2 day job.
As things currently stand, there is no other way to generate pre-tax savings. On the plus side, you will be able to backload your 2019 contributions once you become eligible in June 2019. Had you known about this 1-year delay before starting the new job, you could have front-loaded the 2018 contributions at your old job. This is why it pays to request and go over companies' benefits before signing on.