It's easier for a single leader (Not unaccountable! The moment you start acting like you are is the moment your regime starts to fall apart) to force a compromise.
It's just as easy for said leader to force a solution that he prefers. And any autocrat worthy of the title is by definition unaccountable (Because power rests solely with him. None of this weak, democratic "All power derives from the people" bull****).
That's something you don't seem to get: You cannot have a supreme leader without also abolishing the concept of popular sovereignty. And once you do that, the ruling elite is no longer accountable to anyone except themselves.
With no easy way to kick him out, a smart ruler can placate the populace with measures that do not actually do exactly what they want, but the resulting situation is acceptable enough not to start rioting over it.
And a stupid ruler can ruin everything, something you also completely ignore. At least in parliamentary democracies, there are set term limits for rulers that will limit the amount of damage any single one can do.
He is in no danger from extremist rhetoric that might score points with common people (like it currently does all over Europe), but is otherwise disastrous.
No, he is in danger from extremist rhetoric that will score points with him. Why would a given ruler be immune to extremism?
This does assume the king is smarter than most of his subjects, of course (usually the case with modern absolute monarchies. Yes, even Saudi Arabia the royals are actually the progressive ones. Tells you a lot about Saudi Arabia in general, really).
Even very smart people are not immune to stupidity, extremism, myopia and a myriad of cognitive biases. By spreading the load of governance over a multitude of bodies, democracy has at least a chance to introduce corrective measures before any real damage is done; any single autocratic leader (no matter how smart) is one bad advisor away from inflicting disaster.
But yeah. Do tell us more about how Europe's past experiences with autocracy (Little things like the 30 Year's War, WW1, WW2, a multitude of succession wars all over the continent in the wake of the roman empire's collapse) are not in any way relevant to this new, better Monarchy you're advocating for. The only successful monarchies in the west right now are the ones in which the Monarch's subjects agree to do as the Monarch asks, as long as the Monarch never asks for anything.