> Who gets to be the arbiter of truth?

The same people who decide truth in a defamation case. Let's not pretend that truth doesn't exist or that it's impossible to determine. Anybody can make a factual error, or make a well-meaning post that turns out to be wrong, but that's not what we're talking about here. We're discussing accounts whose entire purpose is to spread harmful disinformation.

> What if you create all this infrastructure for regulating speech, and then the political winds shift and a strongman president ends up using it to suppress speech they don't like?

Again, if the law banning this practice is well written it will be impossible to do that within the context of the law. The fact that some hypothetical strongman president might be able to get away with suppressing speech by acting outside of the law, or might be able to pass other laws that allow for it, is irrelevant. They could theoretically do anything at anytime to anyone regardless.