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It seems that it is the right way to connect to the Ender's main board! - Just like a BLTouch or any servo. Also, I read that you will lose the buzzer signal by doing it as the pin 27 was responsible for that.
Mine is just like you mentioned, common +5v for both servo and endstop, common ground and dedicated wire for each signal. Just be careful because my Arduino+Ramps did not have enough current to feed the servo and I needed to feed from external +5v supply. I do not know about the Ender 3 board but if it behaves weirdly with the servo try an external +5v source with the common ground! I used a hobby "UBEC" at the time.
About the spring and thread you need to find a spring that is almost the same diameter as the head of the screw - if it is too close to the thread the spring can enter the "channel;tunnel;hole" of the screw and lock it there sometimes. I use a standard M3x30 that is not fully threaded. Polishing it, like you did, would be better :D