Your work is a lot better than you think!
As for your “lack of artistic substance”, I think perhaps it may be a case of trying to do too much in one piece. The simpler pieces you have like the hawk (which is by far my favorite and IMO has an ass ton of substance) have a lot more appeal and life to me than the others. I also think that you have attributes of conflicting styles within a single piece. I think generally your style favors characters who’s faces have a little more realism in than what you have drawn. For example take your Strider pinup and cover the bottom 3 characters… Then cover the top part. They feel conflicting to me?
Same with the joystick dude. You could simplify the lines on his body/clothes although the quirks in your line work convince me that upping the detail of the face would suit it even better.
I look at your sketch of the swordsman with the beads, look at his face and arms, and I see that cohesiveness… I dunno if you feeling me here haha. I also think it’s a good idea to thumbnail composition on a scrap piece of paper and then reframe them as you wish. A lot of your pieces feel a little suffocated because of lack of negative space between characters and breathing room on the edges. It feels like your characters are becoming unintentionally shorter as you draw and create a forceful composition because of it.
The first thing people look at on characters are the eyes. Maybe try focusing your line skills and touches on the eyes and face.
Great work…Just keep it up, mayne.