Hi. Thanks for responding to my post. 
I'll answer to the best of my ability.
- For me personally, I was for the longest time like many young artists caught in the vicious cycle of always receiving praise for my art growing up which in hindsight really damaged me as an artist since I genuinely believed I was drawing good and thus wasn't aware of the many many flaws my art style had and still has depending on who you ask. I know the frustration that comes with wading through an endless sea of the same cut and paste tutorials because Sturgeon's Law.
That said, I'm always surrounded by art since I follow both artists I like and general "anime" accounts on Twitter and Instagram since drawing is one of my obsessions in life and as such, my brain tends to absorb certain details in other people's art like a sponge and of course, I make a habit of liking or saving pieces I really like if I think it could help me down the road. This for me was my "ah-ha" moment after trying to go the "mentor" route, but not having the results I was looking for either because we didn't click, they didn't teach me anything I didn't already know, or some of them weren't interested in my plight.
Knowing what to get better at really just comes down to your willingness to critically look at your own work and the most common way to do that is to simply leave it alone for a while then come back to it with a fresh mind. Often I've done that and would notice things like "Oh, her shoulders are too broad" or "The gesture her hand is making doesn't look that good." that I wouldn't have noticed had I just drawn it all (roughs, inks, coloring) in one sitting and even then, just by drawing LoaA so much, I've gotten better at noticing mistakes on the fly during the roughing stage of the process and therefore can quickly fix them without having to take a break and come back later which saves time when you are on a schedule for your comic.
Here are two examples of finding and improving upon flaws in my art all on my own using the previously mentioned method of using a search engine for tutorials and such and the aforementioned method in this post of keeping (and saving) a list of anime art pieces I liked and how it helped me easily acquire a better coloring palette for the latest chapter of my comic without me having through hours of trial and error.
Drawing example: I didn't know how to properly draw convincing breasts on my female characters growing up. Searching on DeviantArt for good breast drawing tutorials fixed that. Particularly, the best one I came across suggested to think of women's breasts as a half filled water balloon with the end pinched. Another way to put it would be to think of a circle, then apply gravity thus giving it a tear shaped appearance which is closer to how most women's breasts actually look irl. (Of course, some may have a more rounded appearance, more saggy appearance, and some are so small that they are barely there, but the same general idea still applies.)
Coloring example: When I started Chapter 6 of my comic, Life of an Aspie, I decided to experiment with the color scheme for my characters since the coloring scheme for Chapter 5 while decent left a lot to be desired. At first, I didn't know what to do so I just hopped on Twitter, scrolled through anime drawings of girls I had previously liked, saved a few of them that I thought had a good coloring scheme, opened them in Clip Studio Paint and using the Eyedropper tool, I was able to sample the different colors the artists had used to create a convincing color pallets. As a result, my colors for my comic right now in Chapter 6 look much better than the colors I myself came up with for Chapter 5.
As for "getting to the summit", I don't think there's ever a point where you reach a "summit" even if you are one of those advanced artists. A plateau sure, but speaking of advanced art, the biggest thing that separates advanced art from beginner art is realism and for that, there are books you can buy, artists with realistic styles you can study, even a simple pose practice website can help you learn the more advanced concepts like dynamic lines and foreshortening even if you just rush the drawing because of time limits.
In fact, Going back to tutorials for a moment, there is one that I found in my niche that really stuck out to me that talked about the different different skill levels of an artist and used an animu girl to help illustrate his point of going from "noob" to pro with the starting levels having the girl drawn in a choppy cartoon-y style that any beginning artist would inevitably draw in without the use of more advanced skill techniques with the later skill levels having her gradually looking more realistic which you would come to expect from somebody who's applied themselves.
2.
The easiest way to notice when a certain character is hogging the spotlight at the expense of underdeveloped ones is how much you physically insert them into the narrative. This can apply even to the main character who's supposed to be front and center of your story. For example, in Chapter 4 of LoaA, I had characters talking about Susan and had waited till near the end of the chapter to have her make a physical appearance. This way, I allowed my other characters to have a little bit of development as they discussed her actions in that chapter. In fact, I created a new character who saw the events of the chapter unfold from his perspective as he wandered through the school picking up bits and pieces about what happened.
The weakest link in a comic could be anything as different people have different opinions, but if there is a general consensus among readers that there is one part that sticks out like a sore thumb, then the weakest link becomes that much easier to find and fix. i.e Starting out, it was some ugly ass tones I was using and stubbornly clinging onto certain that I would find a way to not make them look like ass. Initially, my plan to fix that was to revamp the art of the older chapters completely and add color to them as well, but real life got in the way of that several pages in and I had to resort to simply using de-toning my tones so that they looked more paint like.
Lastly, subs come and go. You'll never really know why your sub count is dropping at times unless its a really bad story decision you made or something where its clear you screwed up on your part and it wasn't simply because the sub got bored or the story didn't click with them anymore.
This post got really long, but I do hope this answers most of your counter argument.
