Plenty of countries that are well-run democracies don’t have separation of powers between the legislature and the executive — the UK is one of many examples.

Separation between the executive and the judiciary is important, but separation from the legislature doesn’t really seem to be.

Even among countries that do have such a separation, the US is unique in making it so difficult for the legislature to pass anything, which IMO is the most serious flaw in its system. The permanent deadlock is what creates such a temptation for the executive to circumvent the rule of law and try to seize power wherever it can.