External insulation ticks many boxes but cold bridging still takes place at the base profile.
A 1.5mm Aluminium base rail has the same conductivity as the 8 m of polystyrene or rockwool insulation above it.
Since heat takes the path of least resistance. The area of wall affected by the cold bridge extends beyond the L shaped mounting surface.
This obviously applies to stop-tracks and over-sills. Average U-value calculations should consider the mounting area of these items.
Consider using Stainless Steel that has almost 1/13 the thermal conductivity of Aluminium.