Most of the blighted systems in US government could heal organically (democratically) with just a handful of critical amendments. Throwing it out might be overkill.

For example, establishing multi member district plurality would allow FAR greater flexibility for major progress (including further amendments to the Constitution) since coalition governments have larger majorities to clear the otherwise stringent voting thresholds.

Re: the presidency, I tend to agree. Beyond the usefulness of a figurehead, every purported benefit of a unilateral decision maker is a potential liability of equal or greater magnitude.

TL;DR: all constitutions can/should be fixed continuously. Figurehead-oriented presidencies can be useful, but one office should wield little to no power. Giving them nukes is a recipe for disaster.