Deals and bargaining all happen AFTER a party takes power and completely hidden until a government can't pass their own bills like the Labour attempt to reform welfare.
With proportional representation the deals are made in order to form a government, BEFORE it has power, and are between separate political parties.
Sure there may be agreements that are not all made public, but these are much harder to keep in the "backroom".
2015 we voted for Cameron, ended up with May then Johnson 2019 we voted for Johnson, ended up with Truss(!!) then Sunak(!)
PMs don't drive the agenda. The UK is one of the most corrupt developed countries in the world. The people driving the agenda are billionaire and multi-millionaire donors.
PM is a sales job, not a strategy job, and increasingly ridiculous PMs have been selected because the donors have had enough of liberal democracy as a concept. If it stops working - which it pretty much has - there's going to be less resistance to removing it altogether.
Which is why there's resistance to Digital ID. There's widespread distrust - with reason - of the political establishment right across the divide.
The point really was about parties themselves being coalitions in all but name.