I do agree that the lack of a headphone jack absolutely kills me. It's a reason I haven't pulled the trigger either way on a new phone yet. On the one hand, I want a secure degoogled phone that maintains a lot of functionality with GOS. On the other, I want a modern phone with a headphone jack a la Sony. I go back and forth constantly.

That's fair, and the reasons why someone buys a phone is a personal choice.

I would suggest with things like a headphone jack that, while its annoying to buy an adapter (usb-c to headphone) it may be worth the cost vs sacrificing something like hardware security.

Sadly a lot of the time consumers are forced to choose between security and privacy or convenience.

That work was not available when GrapheneOS was developed

What do you mean ? This has nothing to do with GrapheneOS in the first place (which by the way has been created in 2014. The article i linked refers to 2021).

I still consider it a black box.

Reverse engineering is a thing. It always has been. If every piece of closed source was a blackbox how can you explain exploitation ? How can bad actors exploit Windows, MacOS, CPU firmware and so on ? Your argument here is not practical. Also, why should Google put a backdoor inside a chip ? They already get every information they what directly from the people agreeing to use their software. So, why bother ? Moreover, every phone on the market has closed source firmware.