Impossible to unroot?

I really need help, I finally managed to get myself a twrp recovery installed on my device everything worked fine and I was able to flash a custom rom. It is rooted and came with SuperSu or whatever. But my number 1 killer is snapchat. And whatever other apps I may come across that detects root. So...I figured I actually don't need root, I just wanted a custom rom and that doesn't need root and I have no idea how to unroot. I tried using SuperSU in settings and full unroot thing and it doesnt let me boot my device after its done, instead it loops the bootup over and over and always ends each loop with a black screen and a flashing red notification LED. All I want is to flash my custom rom but not have root (I think the rom comes with it idk) But yeah I really wanna know how to unroot without wiping data etc. It also asks me if I want to go back to a stock image or something but if I select that in supersu it just says uninstallation failed. Ive been trying everything, or just a way to hide my root to access apps (magix, xposed, xposed framework, hide my root etc etc etc DO NOT WORK) Do I just need to get a different rom or something? Thanks in advance