In most situations to learn some fundamental I’d always suggest going out and drawing from life. Not only can you get a good basis for perspective you can begin to understand depth alongside perspective by showing how objects relate to each other as they get further away. But also, practicing as you are doing by creating vanishing points and filling in shapes is good. Start simple with boxes then graduate to more than one. Create a simplified building. Creat a simplified cityscape. Make a building and build more details in it to learn size comparison (try adding other items - wheels, barrels, chairs, fences, chimneys, sheds, tools…etc.) There are probably loads of videos and ideas you can try from youtube and what not. Back many years ago when I had a short lived stint in a art/design college I remember 2 projects in which 1 we had to draw an interior scene (I drew a bar), and another was a lighthouse in 3 point perspective. You could do that, or really anything your wee lil’ heart desires. Good luck!

Mileage may vary with this advice, but I like doing comics as projects to get better at just about everything. A comic book page offers up challenge after challenge to solve. As you solve those problems, you will level up--page by page, and panel by panel. I've seen storyboard artists recommend this, as well as animators. I make t-shirts for a living, but making comics is where I find I improve the most. If you want to improve on perspective, get a script that has cities, interiors, and vehicles. That will through you into the deep end of the pool. The other part is what you're already doing, which is studying theory. I think Marshall's perspective videos are great. I bought them years ago and they helped me tremendously. Ditto on Scott Robertson's book.

Take a picture of a building that's in one point perspective, Then draw it in two, three point perspective. Then draw un up view, then a down shot, etc. This is a good way to gain control of perspective :)

Can you show us some example of your work or studies on perspective? It's easier and more accurate for us to give advice, if you could provide some things you done.