You appear to be painting indirectly, which is my preferred method.
My best tip is to not get so attached to your underpainting that you don’t push your subsequent glazing layers. When painting indirectly, we tend to get nervous when we see our dead layer change from its original parameters and values, and fearing a loss of detail and form, we may not continue to build up color. This results in a painting that looks colorized rather than realistically natural and intentional.
When incorporating velatura and scumbling, opt for a transparent white- lead whites are most archival, such as flake and Cremnitz, but zinc is also an option (though I avoid it). Transparent whites will keep your colors true and warm, without sacrificing body or compromising details established in the previous layers.
I can’t really think of any specific YouTube videos I’d recommend off hand, but Cesar Santos is one of my favorite contemporary artists who employs some indirect methods and I know he has a YouTube channel. Bear in mind, he doesn’t always work from a grisaille, but more frequently starts with ébauche.
Most of my current painting heroes actually rely more on ébauche than glazing a grisaille/brunaille/verdaccio or any other type of refined underpainting/dead layer. Osamu Obi, while not my favorite style of figurative art, is an extremely talented indirect artist. Alexi Anitonov is another artist that’s known for painting indirectly.
Beyond that, my best advice is simply paint often. Probably the best practice I gained from art school was that of painting from life on a daily basis, or nearly daily basis. Improving hand-eye coordination, learning to accurately render what I saw, allowing style to develop through trial, error, and time, was more helpful to me than anything else.
If you can’t paint on a daily basis, at least draw/sketch. Set aside an hour to draw just a single object. It’s not something you necessarily have to do indefinitely, but it is extremely helpful to acquire this habit for awhile. If you do this, even when you are not drawing or painting, you will be learning from observation. You will begin to notice the way light bends around household items, or someone’s wrist when they are moving their hands, you will notice colors and light in shadow, without even realizing you’re gathering the information. I will sometimes go through long periods of time without painting to take up another medium or to devote myself to something that has nothing to do with art, and when I resume painting again, I have improved from the point at which I left off. Because I have conditioned myself to view everything I perceive with an artist’s eye, even if I am not working with paint specifically. This is easily achieved simply by practicing your technical rendering abilities on a regular basis for an extended period of time.
My last suggestion is- rather than using black from the tube to create your grisaille- mix a neutral black from burnt sienna, ivory black, and titanium white, or preferably use Van Dyke Brown. It will be easier to build on than a very cool black.