Hey there!

It's a little difficult to give you a proper analysis as you only have two pieces up, but I can do my best.
In your post you talked about foundations, and the thing about foundations is that you never really stop practicing them. The reason for this is because they're the building blocks of everything you do in art. Every complex organic shape comes from the sphere and every complicated looking structure can be broken down into boxes in space.
So definitely work at those!

And they can be a lot of things. I usually suggest people start with the sphere and basic perspective. And then apply that information to more complex exercises.
And draw all the time. That's probably the best advice anyone can give you. The more hours you put in the better you will become.

Looking at what you have uploaded, I'd say you should work on drawing with a confident hand. Your lines are 'hairy'. That is, they are a lot of small strokes instead of larger smooth strokes.
Since it looks like you're drawing with a tablet, I might suggest working with traditional media as well. Digital programs are trying to emulate the real thing, so the real thing's sometimes the best place to start.

Working on volumes would help you too. That can be achieved with practice and observing objects in life. For instance, you must have used reference for your tank but not for the barn next to it, because while one feels solid and voluminous, the other does not. Volume is important because it gives us that solid feeling that we're trying to translate into 2d space.

There's a million more things you could work on, but those basics are the bottom line for all of it. For instance, drapery. You'll get better at drawing clothing on a figure through volumes and observational drawing. Same for anatomy.
There are of course more little tricks to learn along the way. Anatomy can be memorized, and fabric folds in specific quantifiable ways. But at the core is the foundations. :)
If you can, find a life drawing group near you. Or you can take drawing classes at a community college or community center.
And if you'd rather continue on your own there's a million videos online to get you started, Doing a cursory search I found a few:
Here, here, and here.

Some of the ones I like deal heavily in anatomy and they are Proko and The New Masters Academy.
I also really enjoy Ctrl-Paint for digital stuff. There is paid content, but the information is worth it and there are a lot of free tutorials as well. Especially the foundation videos.

Hope I was helpful~