I agree with the definition of mastery as having something so ingrained in you to the point where you can play it without thinking about it.
I also believe that in order to improve as a drummer, you shouldn't necessarily seek to 'master' everything you are learning, because if this were the case then unless you can spend 10 hours a day practising, it'll take you 10 years just to get basics down.
I take this approach with my piano practice, as it is more relevant to this topic than my drumming at the moment: Right now I'm working through grade 5-6 pieces, and I have the same approach that I've had since I started. If I were to 'master' every single piece in all my books, it would take me weeks and weeks to be able to move forward, and I would learn more slowly.
What my actual approach is, is to learn each piece so that my timing is good, my tempo is good, but it is totally fine if I can't play it perfectly every time. Why? Because each piece gives you something to learn - different fingerings, different melodies, chord patterns, etc. The goal of each piece is not to play it perfectly (unless you are going to perform it), but to learn what you can from it, and then move on.
So right now, if I were to go back to the level 2 or 3 grade material, I would most likely be able to sight-read it and play it right first time - not because I spent weeks learning it when I was first up to it, but because my technical facility has improved drastically through continually moving forward in my studies.
I believe that this is the best approach to continually improving your skills, and then mastery comes into play for KEY components of what you are learning.
So let's say you've been drumming for 7 years, you have a fairly high capability of single strokes, double strokes, paradiddles, double-bass playing, some independence, mixed hand/foot patterns, and linear patterns. By this point, if you are practicing actively and playing in a band, chances are you have mastered basic 4/4 beats simply because you have been playing them over and over and over for 7 years. It's likely that at some points you've had to focus specifically on particular aspects of these 4/4 beats - such as tweaking dynamics or accuracy - and that is part of the process.
I'm sure my point is as clear as mud, but take what you will
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