The president shouldn't be any more popular or powerful than a Senator or representative in Congress, arguably they should be less so because Congress writes the laws and controls the budget. But the way American politics works, driven by populism and celebrity worship, you'd think the President controls the entire government like the CEO of a company.
Remove their power to pardon and issue executive orders - a President shouldn't have the authority to overrule the judicial or legislative branches. Give them a single. six year term. Make it illegal for them to hold any further government position, then boot them out. The Presidency should be nothing more than a revolving door of nobody bureaucrats, like a Prime Minister.
Also, they shouldn't get Secret Service protection for life. They can have the same Second Amendment protections as other citizens after office, which are supposed to be perfectly sufficient. Or let them hire private security.