As for your post, your point about "gradually breaking down the guardrails" is completely eviscerated by "he President just deciding to use a lot of the powers that Congress explicitly gave to the Executive".

It's not. Some of the stuff Trump is doing is deciding to use previously granted powers. Some other stuff is "doing things that are probably prohibited by law, but can have a major impact before the courts can get around to enjoining it." Some other stuff is ignoring non-legal guardrails that are an important part of functioning democracies (like not having the President directly order the AG to prosecute or not prosecute people).

It's not a binary, where if there exist some things that Trump is doing that are non-authoritarian it must mean that everything Trump is doing is non-authoritarian.

Hitler started off with a failed revolution, did time in prison, wrote a manifesto detailing exactly what he wanted to do, formed a street army, and basically bullied his way into power.

Sure. But he didn't start disappearing people without a trial on day one of his bullying his way into power. He gradually changed the nature of the German government. You had a gradual decline of the rule of law, replaced by the rule of whatever advanced the ruling party ideology. Illiberalism didn't happen in a single day, but developed over a series of incremental acts, changes, and programs that moved Germany from a "rule of law" government to a "rule of ideology" government over the course of many years.