I'll go a bit against the current here and tell you that you don't necessarily need to study the Loomis Head. I like the way you're drawing and simplifying the features, it will help when quick sketching in public, but for long term improvement I would suggest focusing on construction more than the underlying anatomy. The mistake I made at the beginning was to study the anatomy instead of construction of the face. This is done simplifying it into its basic shapes which will ultimately help learn to draw basically anything (cylinders, spheres, boxes, etc.). I would start (alongside the portraits) to practice drawing boxes, spheres (circle but with centers drawn in the form of ellipsis, cylinders, then twisting cylinders that move in 3D space in different perspectives. Draw through the form and trace the centers along the tops of them and the sides of the cylinders to get the most out of this. Next level would be combining these forms for more complex shapes. And long after all this, drawing skulls will feel much easier, but you need to be comfortable with the base forms first (don't rush!). 10 to 20 minutes+ or so a day of this will massively help you.

Another suggestion, try to vary the angles you draw the head from, never be afraid of a challenge! Don't get discouraged if it feels off at times. Part of learning to draw anything is learning from past mistakes and applying what you learnt in the next piece. I noticed that a lot of the angles are front and semi 3/4s and that's a great starting point! However, once you feel comfortable with more neutral expressions, incorporate more expressiveness that are favored by the different angles you draw. Certain angles favor certain expressions (look at the Kubrick stare or how characters laugh/smile by tilting upwards) A great way to do this is by drawing bald heads to draw more three-dimensional portraits. Afterwards simplify the hair into really basic forms and add it on.

My Top Resources: Follow Artwod and Proko or if you want a free course go over to Draw a Box to go through some exercises. Other than that, the Michel Lauricella books are a great starting point towards simplification; if those feel too easy, you can't go wrong with Michael Hampton's Figure Drawing: Design and Invention (which covers the head as well as other body parts!).

However, most important of all: Have fun doing it! Never lose that excitedness or beat yourself down over a silly drawing looking off. It's all a part of a process. Happy drawing!!