Maimai already redlined it, but yeah, chest is too high. Body looks smaller compared to her head, which would be bad for her since the weight of her head would tip her balance off. Hand -- mostly the thumb. Needs to be longer, and holding onto the wrench. Hard to hold the wrench, or anything for that matter, without your thumb.
Also, about the study anatomy part. Contrary to what you said, I actually do know what that means when people say it. I guess what you're trying to say is it's too vague of an advice since there's so much to study about anatomy. So I'm going to say it, and elaborate on it. Study proportions (like how long is your upper arm supposed to be compared to your torso -- answer's actually your elbow should be at the waist, and your arm is as long as your legs), muscles (which could help you with how to draw hands), and bones/skeletal structure (pretty much helps with everything, like trying to figure out why that arm looks like it's bending awkwardly, stuff like that). Start with anything you want, you think you need help on the most, or you draw the most. You can start by studying the torso, like where the clavicle starts, and where does it rest, how does it look in different poses. If you stretch your arms upwards, what does it do? Or how does your pectorals look when your arms are stretched. Stuff like that. It does take a while to get these things, but it's worth it in the end, imo.
Another is to up your contrast with the shadows. A way to check this is to view it in grayscale or black and white (you're using SAI so it's going to be the luminance to transparency one). Because you don't use lines, I can barely distinguish which is which. So you'll want to work on that.
And as for working too much on something: I don't think you should be alarmed at how you work. The time it takes to work on one piece is a work-in-progress in itself, just as your style is and all that. If you want to lessen the amount of time it took you to do this, find shortcuts, or combine a few steps. It's really up to you since this is your style. Also, this is just what I've seen, but you don't really make amazing art in just 10 minutes, so take your time on that. If you still feel bad about it, go look at some amazing artists' speed paints and notice how long it takes for them to work on one piece. I recently watched shilin's speed paint
on one of her drawings of her Carciphona characters, and the whole video was more than two hours. And that's just sped up. The whole thing took her around 20 hours of working on the piece, so don't be alarmed. I've worked on a piece that took me more than 20 hours
myself (though I think that's mostly because my laptop is shitty as heck, lagged a whole lot). Though for me, it only took me two days because I barely ate anything, just worked on my stuff, and only stopped to go pee, when my stomach grumbles, or sleep. I worked on it non-stop during the day, and throughout the night as well.
There are tons of pixiv artists that work the same method as you do. What I've noticed though is they usually don't do very detailed, very clean pieces when they work this way. It's usually very messy, so that's probably why it took you longer.