what you are describing is a perfect symmetrical, isometric cube, which has no vanishing point and thus is impossible to actually exist. What you are trying to show is that the the cube is not perfectly set up like this:


In a correct cube with a vanishing point. those green and pink lines you drew, should not be symmetrical, they should go towards each other.
For example this cube:

You see I drew all the same lines you did. and these didn't math either.
Only if I wanted to draw a perfectly symmetric and isometric cube, these lines would match up.

Your char design is really boring, even if its just a normal guy, he needs something to make him stand out. even just a shirt with a text on it could be enough.

I don't really have much to say about the art, it just looks kind of unfinished without the shading.

I fixed the line to actually go to the perspective and made it lighter, like you said.

The problem is that it doesn't look like he thrust the spear, more like he just extends his arm like he wants to push something over.
So firstly that means you have to remove some frames before the attack. What is also very important is an anticipation frame,
 your animation already kind of had this, but it didn't really work out well since he never returns his spear. Then you can also add a cool effect where you have 1 frame where the spear goes further than it should go, this makes it look a lot more lively and active.
Example:


If you wanna be a really cool dude you can add a SMEAR. this might not always work so be careful when you apply it, but I think it works here.
Example:


Here is a picture of both spritesheets:


Hope this helps a bit.