To the point about your linework - as a whole I'm not too worried about what I'm seeing here. There is a bit of a tendency to draw your original masses more faintly than those that follow, which you should work on, but as a whole your linework certainly is confident.

As a whole, you are indeed doing a good job. Your constructions feel quite solid, you're employing the principles of construction fairly well, and you're demonstrating very strong observational skills, and the patience to employ them properly. There are however a few issues that stand out, which I want you to be more mindful of.

The first point that I caught is that in a few specific cases (usually due to the slight looseness of some of your ellipses), you end up cutting back into the silhouettes of the forms you construct. This doesn't happen often, and you have improved a great deal on this front from lesson 4, but it does happen. For example, here in your buffalo's legs, here in a toad's head, and so on. In the circumstances where you end up with an ellipse that ends up loose (from drawing through it), that's perfectly okay, just be sure to treat the outermost perimeter of that shape as the silhouette of your form.

Next, I do agree that you are a little too aggressive with your line weight, and you need to tone that back. Line weight is itself a specific tool, and serves a particular purpose. It is to clarify how specific forms overlap in specific areas, and should only be applied to limited, localized areas of existing linework. It should also be applied using the ghsoting method, rather than traced slowly and carefully back over the existing lines (as tracing can tend to cause us to focus too much on how the lines we're drawing sit on the flat page, rather than how they represent edges in 3D space). Furthermore, do not use line weight as an opportunity to alter the silhouette of a form in your construction. That's another risk that comes about when we trace back over existing lines. Sometimes this can cause the "bumps" we get from adding additional masses to get kind of smoothed over, almost as though we're wrapping the whole body in a sleeve or a sheet. Those bumps and the little valleys we get where they overlap should be preserved - they help promote the impression of musculature on the animal's body, and only come about from trying to wrap one form around another.

To that point, the next issue I wanted to address was how you approach your additional masses. Right now the silhouettes of those masses appear to be somewhat arbitrary, and quite amorphous/blob-like. One thing that helps with the shape here is to think about how the mass would behave when existing first in the void of empty space, on its own. It all comes down to the silhouette of the mass - here, with nothing else to touch it, our mass would exist like a soft ball of meat or clay, made up only of outward curves. A simple circle for a silhouette.

Then, as it presses against an existing structure, the silhouette starts to get more complex. It forms inward curves wherever it makes contact, responding directly to the forms that are present. The silhouette is never random, of course - always changing in response to clear, defined structure. You can see this demonstrated in this diagram.

This essentially means that instead of drawing them as blobs and figuring out how they integrate with the structure afterwards, we have to draw their silhouettes with a highly specific set of curves - placing outward curves where they aren't touching anything, and inward curves/corners where they are pressing against parts of the body. Here I've outlined some spots where it was done well, and others where it could have been done better.

The last point I wanted to share is in regards to head construction. Overall you're handling this pretty well, but I recommend drawing your eye sockets to be larger, and make them pentagonal, with the point facing downwards. You can see this in action, along with a more detailed explanation on how to think about head construction in this informal demo. This will eventually serve as the basis for the updated head constructional material for the main lesson, but I won't be able to revise that content until I'm done revising all the lessons preceding it.

While overall I do feel you're doing a pretty great job, I would like to make sure that you are able to apply what I've listed here. I'm going to assign a few additional pages of revisions below.