I forgot about your needle wheel pump and venturi setup. Im still of the opinion that fluming will work better, but you can test that I guess.
It occurs to me that you could put a diffuser of some sort on the pipe to spread out or reduce the pinpoint pressure on the roots. As long as it didnt reduce the over all flow too much, you should still get plenty of surface exchange - which is the main goal with fluming or water falls. Its the constant refreshing of the surface water that causes the increase in the rate of DO. You dont really need it to lift the surface in one spot. You just need to exchange or replace the surface water with water from below- the faster the better.
You could do a simple DIY manifold thing - install a 'T' or 'X' fitting where the pipe enters. Then put lateral pipes coming off that fitting with lots of small holes. That would spread out the force over a much larger area and might not reduce the flow rate too much. You would still get the same exchange/refresh rate at the surface of the water as long as the flow rate is the same.
Here is a crapy drawing , but red is the pipe, black bucket and the black sloppy circles are holes
You could even keep all the holes on the sides and bottom of the manifold to reduce root swishing even more. With enough holes, you shouldnt reduce the flow much at all and it will spread out the pressure even more.