If you HAVE to get the starter running off the ECU good luck.

But, if your not so fussy don't go over the mountain, go around it.

Run the starter straight off the ECU battery via a button in the model,
just like the old days & like I do with my manual start Wren.

It should not take much thinking to find the cut off speed of the starter
(listen for it when you use your friend's ECU & note the RPM on the GCU.

Of course you will have to do the cool down cycle yourself as well.

Will the ECU let you start like this? - John.