Where along the cable did you put the cable brake?
On my 340, the cable is held securely all along its entire length from the engine back towards the throttle. Except for about a 1 foot wide loop just under the throttle inside the pedestal. The loop in the cable allows it to have a smooth arc to enter the bottom of the pedestal and into the boat.
That’s where the cable brake should be located.
The brake will stop the loop from bending every time you move the throttle handle. At least on my boat, its that loop the caused my handle to drift back and never allowed the handle to stay in place.
Very annoying and extremely tiring while motoring for any length of time.
It may not be a complete circle type loop you should look for, but more of a wide arch of cable that is unsecured. Most likely in the same area I described.
Once the brake is in this area you’ll only need a minimal amount of pressure on it to get the results you want. And not have to worry about crushing the cable.
When you do find this type of loop, move the throttle handle back and forth and watch the movement the cable has each time. Attach the brake and look again to see the difference.
I hope this helps you.