low spoke dynamo hubs in 15mm thru and centre lock don't exist.
There's a reason for that :-)
If you persist with the 36 into 18 project, build it 12:6 with the 12 spokes on the disc side, that's the lowest number I've seen being tasked with transmitting brake torque to the rim. Since the angles of the spokes will be slightly different between pushing and pulling, arrange it so that the pulling spokes (when braking) are the more tangential.
There's a reason for that :-)
If you persist with the 36 into 18 project, build it 12:6 with the 12 spokes on the disc side, that's the lowest number I've seen being tasked with transmitting brake torque to the rim. Since the angles of the spokes will be slightly different between pushing and pulling, arrange it so that the pulling spokes (when braking) are the more tangential.