Shoot them
We're allowed to kill Abe Lincoln and JFK, why aren't we allowed to kill these guys? Why does Reagan get plot armor out of all the presidents? Who wrote this shit?
To be fair JFK getting shot was pretty epic though. He almost caused nuclear holocaust (although a more rabidly anti-communist president may have definitely caused nuclear holocaust), war crimed the South Vietnamese a ton, and stabilized Israel. RFK getting assassinated was less epic because that gave us Nixon
Why would we have a PM? We already have the Speaker of the House and the Senate Majority Leader. And the PM wouldn't be able to exercise any executive powers unless you burned the constitution, because separation of powers.
There's not really a good reason to adopt a parliamentary system like the UK's for example if we were to completely reform the government imo. Or to have a PM separate from the president at all.