I did mostly what you did at first and was on the verge of giving up; however, instead of applying external forces onto the flipper such as Add Impulse and Add Torque, I use the Physical Constraint’s Angular Motor. So far it seems to be working out well for me, but before I get into details about it, I would uncheck *Enable Projection *(as it may make the flipper fly off) and check mark Disable Collision under Constraint Behavior.
Back to Angular Motor, I set the Angular Drive Mode to Twist and Swing, kept the Target Orientation to 0, 0, 0 with ***Drives ***to have only *Swing *check marked. For the Strength, you need to set it so when the ball collides with the flipper the flipper would not move under the ball’s influence. From my understanding based on my experience with playing around with Angular Motor, Target Orientation is essentially trying to hold the flipper at whatever position you set it to be based on the given amount of Strength. Target Velocity is the direction and orientation speed that goes against Target Orientation with *Swing *enabled, you can set the amount of ***Strength ***required to turn the flipper.
Bottom line, look into** Angular Motor**, it does a better job than Add Torque and Add Angular Impulse.
I have a video presentation here -> https://cdn.discordapp.com/attachments/221799426820276226/815014657550581761/vlc-record-2021-02-26-19h12m15s-FlipNFlop.mp4-.mp4