The slow-mo is good stuff. Really surprised at the quality of the video.
If you look at your serve, I think you can see that you're hitting into the ball. To brush, you're going to need to pull/push along the ball, right? So, how are we going to accomplish that?
First off, I think you should really change your racket angle. You've probably seen Fan Zhendong, Zhang Jike and whatnot serve with a very vertical racket, and you're trying to copy it. I suggest you adopt a more horizontal racket, because brushing is tremendously easier with that method.
Hold the racket level to the horizon with your serve grip, throw the ball up and try to time your slice so that you brush the ball. Keep doing that, and do it everyday, all the time. Keep your wrist and arm relaxed, and practice brushing when relaxed. If you feel you don't have enough control and need to tense up: don't. You will be a little bit better tomorrow, and it'll get easier fast.
Do simple serves onto the ground and try to get the ball to come back to you. Don't worry about the ball bouncing too high for now: just focus on spin.
Get a marker and draw a line going along the ball, then color one side with the marker. This will let you see the spin pretty clearly. Try to achieve a very pure backspin in your serves. In match play you're probably going to want to add a sidespin, but you're not gonna be able to do that consistently until you can produce just backspin.
Chances are, most of the time you think you're serving back or topspin, but it's really just sidespin. You will understand the timing for different spins as you serve more with the colored ball.
Start easy and simple, and focus on trying to serve heavy first, then control that spin to get the placement you want.
Don't rush it: I've been serving at the very least a hundred balls a day for two years straight and I'm just now getting my serves to a level where I'm somewhat satisfied.