Along with other issues you see, if you can help, I have trouble continuing to draw after I get the basic figure down and have to move on to the finer details, I don't know what in particular to work on anymore.
Feeling restrained by your canvas is a legitimate thing, but it's not one I'm sure I have tips for.
As for struggling to move forward and incorporate details, that's probably the result of not having a goal set for yourself. "Draw the thing" is a bit of a broad concept, so it might be good to switch it up and narrow it down.
Are you happy with the proportions? If not, spend a week or so focusing on proportions.
Are you happy with the motion and fluidity of your getures? If not, spend a week or so focusing on drawing short 30s gestures in as few lines as possible.
Do you feel capable of drawing faces? (I don't actually see any face details in your gestures, but one guy's underwear is surprisingly detailed.) If not, spend some time doing face gestures instead.
Drawing without a goal is not going to improve your skillset, it typically just stalls you and becomes demotivating. There are some things you can work on, but how you work on them depends on your goal. Pick one, and you'll have a much easier time moving forward!
When I don’t know what to work on, that means I’m done. Seriously.
The antidote for this done feeling is to look at art that isn’t mine, take photos, or look over my personal projects until I find a goal so that I can see where to go from the old point of feeling done. For most artists, figure study isn’t the end goal, it’s a discipline so they have the skill needed to do the real thing.
Maybe you want to get faster so you can crank out panels for a story in comic form. Maybe you want to draw fan art for a favorite TV show where you’re drawing stuff that never happened. Maybe you want to do urban sketching where you’re not skipping out on drawing people. The goal doesn’t have to be high brow or something suitable for an art gallery, it just has to make you happy.