The setup is now as follows…
QU16 connected via ~20m of cat6 (two patch cables and one infrastructure) into a switch.
Vlanned (over a single fibre) to an identical switch, through another set of patch/infrastructure/patch cables to the AR24.
The fibre is about another 15m strung across the road.
The same switches are handling a low latency bidirectional video feed, now tied to the other fibre (it was less fussy about the aggregation).
Without the switches trying to do aggregation or protocol discovery it works – just as it should do.