Luckily I use the deadlifts as straight leg deadlifts FOR the hamstrings so... whatever
You will get tons of hamstring work, in the way they are supposed to work with a proper rdl. What you said right there identifies your problem perfectly. "I over arch and dont know why, but im gonna keep hammering on that movement pattern and strengthen it so I have no chance of doing it right."
Thats akin to using the barbell row as a bicep exercise.
You CAN fix it, and getting to neutral takes practice. Try flexing your abdominals without shortening the torso. Try to shove your pelvis through the end of your glutes as you go down ie. "turn the pelvis upside down". Thats not possible obviously but given that cue will help a lot in getting to true neutral where you are driving from the correct muscles.
The problem you have is very common, especially with people who have been lifting for a while. So much emphasis is put on "chest up and out" that it gets overdone, and eventually takes the glutes out the movement. Start with an unloaded rdl, go to the bottom, and see if you can contract your glutes. If you cant, go up again and "pull down" with the hip flexors. Keep at it until you get the glutes firing hard, this will be the position you are looking for.