I already use ball bearings (608zz 8x22x7). Hrringbonegears would cause alignment problems and currently also not necessary (just my experience) .
I personally don't like PLA, you can't rework it and also no acetone molding.
To the motors, and end stop. I'm working on rotary encoders for the arm and by replacing the steppers with more sutable motors. I played a little with the complete arm and the bottom motor needs at last 2Nm torque. I'm looking for alternatives. With the planed encoder I'm free to use any motor, no need for the inefficient steppers.
But thank you for your ideas and support.