I believe there's this thing of.... teaching yourself to learn, which sounds kinda weird, but it's something that will help you improve.
So, as @nathanKmcwilliams said, copying the work of your idols for teaching purposes is good! And you need to develop this mindset where you're not just blidly copying, but asking yourself, "why did they draw this thing like this?", "Why did they color this thing like this?" and "What is the thinking behind this design choice?" This doesn't really come naturally. Learning how to do master studies is a skill in itself, really.
Knowing art fundamentals such as anatomy helps too. You dont need to be great at drawing realistic anatomy, but when you know which muscles and bones affect the human form, you'll start to understand how other artists interpret and simplify it, and you'll be more able to apply their knowledge onto your own art!
But how do you get better at these things? Practice, tbh. Gesture drawing (line-of-action.com is a good resource), master studies (choose art you think is really good, and pick one special thing or technique within it that you want to pay extra attention to), anatomy studies (look at muscle/skeleton diagram. Draw them. (Gonna sound a bit old school, but for me it really helps to learn the name of different muscle groups, it helps me remember that they exist and affect what the body looks like)).
And for issues with the flatness you're experiencing... Maybe you want to work on pushing your values and line weight a bit more? I really like your art style btw. But you can start experimenting with making darker backgrounds for lighter characters and lighter backgrounds for darker characters, or similar things. Or, for example, see how the last image you posted has pretty desaturated colors on the character and a VERY bright red background? You could experiment more with saturation like that too!
It maybe seems like that's a lot of stuff to do. Maybe you can dedicate an (or a few) hour(s) a week just for learning. You will learn just by drawing too, but making dedicated study classes for yourself, even if they can't be every day or even regularly, will definitely help. I myself notice a pretty big difference in how fast I improve depending on if I have time to study art or if I'm just drawing.
Ahh, I wrote a lot. I really do think your art is great though. I'm just passionate about improving, lol. Good luck!