Why doesn't the successful deportation of Kilmar Abrego Garcia without due process rise to this level for you? It's true he wasn't a US citizen, but he did have a protected status that let him live and work legally in the US. And given that he was deported without due process, but simply by "administrative error," there was no point at which he was given the opportunity to bring up his legal status. That is, the thing that would be different if they tried to do this to a citizen is that they would have successfully done it to a citizen. Presumably the courts would order them to bring the citizen back, but they've already done that with Abrego Garcia, and the administration isn't complying.
If that's your bright line, maybe check out the boot that's straddling it.
Tell this to Kilmar Abrego Garcia, who was here legally, and was grabbed and sent to a prison in El Salvador. The government isn't giving people any opportunity to prove their status. That's what we mean when we say they're not being given "due process." There's not a trial or anything. They're just grabbed and hauled away.