I know exactly how you feel. A cupped left wrist at the top has been my main swing flaw for decades, and I know just how hard it is to fix. I've watched ALL of the YouTube videos on the left wrist at the top and I can promise you that virtually all of the instructors say it is much better to have a bowed or flat lead wrist at the top than a cupped one. Cupped is definitely more of a problem than bowed, Why? Because cupped means the club is open and bowed means it is closed, and virtually all coaches would prefer you to play with a closed face than an open one. Yes, some pros can play great with a cupped left wrist, but they are 1 in 1M.
So, I struggled for years trying to fix it. I would take practice swings and think the club was laid off but it was still slightly across the line. What I have finally done to fix it (somewhat) is to get the feeling that the back of my left hand is pointing directly behind me (on the target line) at the top. It's not, but that's what it feels like, and it gets me much closer to being flat than I ever have been. But this took lots of time and patience. Like swinging an 8 iron at 1/4 speed a thousand times before even putting a ball down. Then hitting 1/4 speed 8 irons for days before going to half speed and then longer clubs. And using lots of video for visual feedback, because it doesn't look like what it feels like. It's probably one of the hardest errors to fix.
I recommend finding some FEEL at the top that puts your left wrist where you want it. Then repeating the backswing a zillion times, always trying to get that FEEL while also using video to ensure that you are actually in the position you want. Then eventually take that feel to the range and start with really slow swings and try to make contact with the ball. Stick to that FEEL and don't give up. Keep at it with the faith that you will eventually get it. There's no quick fix if something like that is engrained in your mechanics.