I just realized I missed 1 rail on each corner of Star Loop, just before the down ramp of the inner lane right-turn.
However, after removing them, the testbench score gets around 2 points higher, which is extremely weird as that extra rail does nothing, and shouldn't interfere with the train pathing ...

I finally improved my previous intersection, adding the last pair of direction/lane pair that was missing.
It can probably be improved on, but at least it makes for a reasonable 4 lane 4 way intersection baseline.
It fits exactly in 6x6 chunks

Blueprint: RHT, Size: 192 x 192, Spacing: 4 tiles ...

I made a probably overkill 4 lane 4 way intersection. The blueprint includes exchangers as the right lane can't turn left (RHT).
It is signaled for 2-4 trains, but there is enough room to signal it for bigger trains, though the exchangers would need to be extended.
It was designed to be exactly 9 ...