Is 3d printing really the most efficient way to make such a structure? Could you not pack straws together and then squish them? Could you not make a corrugated sheets like cardboard?

Mass 3D printing an item that can already be manufactured through e.g. injection moulding or assembling standardised parts is never going to be efficient at scale. That's not what it's for. Where 3D printing excels is at producing one-offs or short runs, immediate production (no time consuming buildout phase manufacturing moulds and setting up production lines), on-site or mobile manufacturing, or making designs that are not amenable to traditional manufacturing processes such as objects with complex geometries or internal voids.

Honestly, cellulose is one of the least desired materials I'd want my house made of. After all the additives needed for making it good enough, it will be some kind of ceramic already, so why not start there?

We can already 3D print concrete. Centralized manufacturing still beats that all hands off... And people still insist on manually assembling bricks for a lot of reasons.

Could we build spaceships with that ?

The main factor when designing a spaceship is mass; getting mass into orbit is currently very expensive. You want to use materials which have very high strength to weight ratio, such as aluminium, and carbon fibre.

Can we build a space elevator with this?

Not with cellulose, and likely not from the ground up. Space elevators are typically imagined as being bootstrapped from a starter cable lowered from orbit. And the tensile strength for a safe one is an order of magnitude or two higher than good steel, to say nothing of plant fibers.

Could we build a space elevator with artificial silk?

https://www.slu.se/en/ew-news/2017/1/spinning-spider-silk-is...