Trustworthy in the context of a "secure as possible" open-source design. Knowing that the layout I shipped is what I got back with no modifications.
Unfortunately my idea would have both the layout and the (OTP-fused) firmware fully open. I'm learning a lot about the challenges of "transparent security" as opposed to security by obscurity...
For the prior, I imagine going there and observing QC procedures is the only hope. For the latter, I imagine it involves keeping discrete critical components isolated -- firmware from silicon (and beyond), using different factories so that neither independently has a functional device.