It’s not a violation of the First Amendment. It is a curtailment of someone’s freedom of speech.
> an official's personal account should only be for personal posts. If the government they are officials of wants an official account to post official statements on, they should set up a separate one solely for that purpose
These rulings effectively say that if you taint your personal account with official business, you have to treat it like an official account. That should lead to a best practice as you describe.