Congratulations on completing the treasure chest challenge! When it comes to the construction of each one, I can see that you are largely applying the principles of perspective correctly, in terms of the steps you're taking and the tools you're using. There's no doubt areas where your skills are still continuing to develop - especially when it comes to drawing some of the larger ellipses to determine the "angle of opening" for those chests, but this will inevitably progress with practice, and a great many of them were handled quite well despite being freehanded. While there were ones like this which ended up being quite far off the mark, the vast majority of them show remarkable aptitude for executing those ellipses.

Generally with this challenge, the bulk of my critique focuses on the designs students produce with the more detailed chests, and I do think there are a few things I can recommend in your work to help you push these farther.

The first thing I wanted to call out is the importance of giving every new piece that we add to a design or construction its own independent thickness. You actually do this a fair bit earlier on in the challenge, as seen here, though you don't even have to give them such a significant amount of depth - just a tiny bit of thickness works wonders. Later on in your set, as shown here, there's far more designs that have those boards/brackets/etc. showing up as paper-thin planes, rather than forms of their own. We can even see some smaller cases of this same issue in the keyhole of the example where you were including thickness in other areas. There are all kinds of things that'll easily go unnoticed by you, but can make a world of difference to the resulting design.

These cases of missing depth/thickness also result in longer, unbroken straight/flat edges in your silhouettes, which is a missed opportunity to make your designs more engaging and interesting at first glance.

Another useful thing to keep in mind is that design is driven by asking ourselves questions. This is actually something I talk about in this video for Proko's youtube channel. When you've got two boards that stick together, a good question to ask yourself is "how are they secured together?" There's all kinds of options, some of which may or may not apply given the context of what you're designing. For example, welding won't work for wooden pieces, and if our chests are older/more ancient looking, hidden adhesive may not be an option.

As shown here, that chest is a great example of one where we've got a top and side planes, but no clear sense of how they're holding together. If it were a solid block of wood, then that'd be fine - but it's a container, with void space in between. It can be as simple as just having some nails that are driven through one board and into the other, or you can go with brackets (either simple or ornate). Lots of opportunities, and once you decide on a route to take, you can dig up plenty of reference to help make those decisions. This is also one way in which reference can be put to good use - not by having it make decisions for you, but rather by helping provide you with options once you've made those kinds of choices.

Reference does of course play a huge role in design. While with practice (that is, practice working from reference) it gradually seeps into our visual library, allowing us to create more complex designs on our own, it's critical to expose ourselves to lots of reference material, and to leverage it as tools in our own designs and drawings. That's the best way to absorb them into our visual library.

The last thing I wanted to suggest is to think about things like wear and tear. When it comes to texture/details, it's easy to feel a little lost in terms of what you can or should be putting down on the page. Obviously things like wood grain are low hanging fruit, but beyond that, it can be difficult to find additional details to add. Ask yourself again, a question - what kind of life has this object lived? Has it been heavily used over the years? Has it developed little nicks and scratches? How resistant to wear are its materials?

Also, remember that detail doesn't necessarily go everywhere - just like everything else, it's a tool. We place detail where we want to create areas of interest, where the viewer's eye should gravitate to. Applying it evenly across the entire object leaves the viewer to figure out where they should be looking. Instead, think about it as though you're orchestrating the manner in which the viewer experiences a drawing - whether it's of a whole scene, or of a single object. The more contrast/visual noise you place in an area, the more it'll draw attention.

So! Again, congratulations on completing the challenge - hopefully my feedback will help give you an idea of what to work on next.