Weird Deformation On Spine Rotation

Hi. I would like some help with this weight paint/deformation problem I’m having. Sorry if I sound dumb or clueless, but I’m new to this software. I think I’ve only been using it for a week.
Anyway, back to the issue at hand. So, I did a bit of weight painting, and I think I slightly got the hang of it for some of the basics of it. The problem is, when I rotate the spine, some weird deformation happens.

Now, I haven’t gone to weight paint the clothes, hair, eyeballs, teeth and tongue yet. I want to take it slowly, bit by bit, so I want to weight paint the body first before getting to any other part.
As you can see, the fingers, arms and head deform under spine rotation. The chest deforms a bit, too, but it’s less obvious.
Now, I weight painted the model, so that everything over the specified joint would bend accordingly to the joint. I did something similar with the pelvis joint, here’s a screenshot of it.

As you can see, everything rotates as normal, except for the eyelashes, teeth and clothes of course, and somehow nothing deforms. I thought if that’s how the pelvis rotates, the spine should be more or less the same. However, that shows that it’s clearly not the case.
Here’s the weight painting for the pelvis joint.

And here’s the weight painting for the spine joint.

I’m not sure what’s wrong here. I could some advice, help or tips. I did try the Fix Deforms function, but it made the weight paint slightly weird. A few spots were blue, and a few were red. However, the deformation was the same nonetheless. I could really use some help, because at this point, I’m kinda running out of ideas to fix this.
I see people requesting the Blend file, and people just uploading the file for others to take a look, so here it is.
Human Meggy.blend (3.3 MB)

…can someone…