Hey You are quite diligent, wow
In case you want to return some day... maybe this one helps?
https://sketchfab.com/3d-models/loomis-head-d0b3f4aa633a44d8bda8cfe2f779f1f8

I think those are good warm up exercises! Even if its just a warmup, pay attention to the perspecitve of your objects (especially the cylinder in that case)
The lines you put on it dont match with the initial circle...
Cheers

You're making a lot of progress! I can definitely see a lot of improvement.
I think one of the main issues is that you're trying to go too fast. Try to slow down in the beginning and analyze what you're doing. You're talking about working on the mouth but you still have questions about the basic head structure and nose.

Aside from that, here are a couple of points for the proportions:

1. It's great to see that you've started drawing the cross, your circles look a lot better. But be careful because you're mistaking it for the eye line. Once you draw it out, erase it and then start drawing the lines for the head.
2. Remember the line in the center of the head indicates the middle. This includes the middle of the chin. It took me a while to realize this when I was learning...
3. Follow your guidelines. The nose should be within the lines.
4. Remember the side of the face is flat, so the edge of the circle should be cut.

Your noses aren't that bad, it's the construction underneath. I took the same nose you drew and placed it in the correct proportions.

It might help to draw over a few pictures of heads like you did with the body earlier.

Hope this helps!

The best way to get better quickly is to fully understand what you're doing while you're learning. If you do all this practice now, but it's full of mistakes, you'll be wasting your time because sooner or later you'll have to come back and re-learn everything correctly.

Practicing different elements correctly one by one in a large volume like you've been doing will benefit more than rushing through topics without taking the time to understand how to do everything. Hopefully that makes sense

For circles, I think it's best to practice with a guide until most of the circles you do are close to perfect. Then practice drawing without the guide until most of them look perfect. While doing this, go back and correct them. You could also do

When drawing the heads, don't draw the guidelines until the circles are close to perfect. I'm sure you can tell when a circle doesn't look right, so just erase and correct the mistakes until it does looks right.

I don't know exactly whatyour head drawing practise looks like, but I do agree with Lesley, that slowing things down, will most likely help you a lot.
I have the feeling that you are trying to do everything at once (basic form of the head, facial features, etc.).
It might help to break down this big skill of constructing a human head, down into smaller managable tasks, so you can focus on one thing at a time and do it well.
For instance, what does drawing the basic form of the head entail?
- being able to draw a circle (great, focus on that and only that until you consider it good enough to move on)
- determining a side plane for this sphere
- drawing the center line of the sphere
- attaching the jaw
- drawing in the nose, hair and eyebrow line
- getting the proportions of the head right

See there is a lot of stuff you could practise

I can only speak from my own experience and I do not practise nowhere near as long as you do, but I focused on these skills mentioned above drawing 2 heads max within 60-90 mins every day, continually comparing it with a reference image and even measuring out the proportions. Its the reflecting, the thinking about what you've done and the comparing with reference that makes you LEARN things you're practising.
Volume is important, but making the same mistake over and over again, because you dont think about what youre doing is a waste of your time...
Work hard, but more importantly work smart, mate!
You've got what it takes!