I strugged with the racket drop on my serve for quite a while, not unlike some other posters here (JackB?). I would have good racket drop on a shadow swing, but the old muscle memory of shallow or no drop would kick in the moment i stepped to the basline and tossed the ball.
I stopped obsessively recording myself in slow motion and watching the video every few serves while practicing and decided to practice for a couple of months without recording. It now seems like I have a good (not great) racket drop compared to before.
Here's my recording from this week:
Here's my serve from last year at the 9 second mark:
And my serve from 6 months ago at the 31 second mark:
I'm happy that I've made progress without obsessing over the racket drop. Reading on these forums probably helped more than the online courses i'd purchased. i forget who, but the person who called it racket-leave-behind as opposed to racket-drop probably deserves the most credit.
If you can point to anything obvious i'm still doing wrong(in the first video), please let me know!