>You'd have a huge explosion but not a classical pyroclastic flow as there's no steep flank.
Pyroclastic flows are caused by the collapse of the "column" of hot gasses carrying ash and rock high over the vent, cooling and contracting enough that gravity overcomes the updraft caused by the tremendous heat of the eruption. It does not require terrain or steep flanks, though they are often present from previous volcanic activity.
An eruption can have many pyroclastic flow events if sufficiently energetic. Those direction of those flows can be influenced by weather and terrain and can travel miles as the collapsing column collects more, hotter debris as it comes crashing down, riding on a raft of superheated gasses gaining strength from the vegetation being vaporized by the heat of its passage.