@art-of-b "I think I'm still at the level where if I'm breaking a rule it's because I'm unaware of the rule"
Then, make your own rules. Like "in my world, heads are going to be square or my mountains have three peaks or my skies are turquoise or everything has this texture..." Not randomly, of course, you look at your work and choose the things you really like. Once you do the same thing over and over people stop telling you that that is incorrect and start telling you "you have style". They can like it or not, that is a different subject. And in the process, you improve. If the thing you choose doesn't really work, you understand that and change the necessary to make it work. No idea is good in the first place.
Some things they advised you I don't agree, some overpainting I feel like yours was better, like if they try to resolve a problem rationally but this is art and the most important thing is that the image makes you feel something (giving more room to breathe to your subjects when they already have a nice separation with the background, and if that were not the case, there are a lot of tools for that purpose. You don't have always to surround your subject with negative space. You can blur the background, desaturate it, if you have horizontal lines in your subject, for example, you can put vertical lines in the background, color contrast, color temperature, size of texture... there are millions of ways. Making a radical change like adding negative space around your subject changes the complete overlay, the complete composition. Perhaps you really want to transmit that your subjects are in an uncomfortable or claustrophobic moment, perhaps you are really interested in cutting the line of the mountain because was to long and it was dragging a lot of attention before, perhaps you think that giving room is a good idea but in your layout is going to make your subjects smaller and that's not acceptable for you, and you decide that is not so important problem and prefer to finish your work and go to the next instead of investing hours in changing everything... Millions of things...). I don't say that neither of these things are the case here (I'm only making examples), but are the things that I consider before saying, "that subjects can be benefit for more room". I'm not saying is a bad idea, I'm saying is a different thing that you can do in your drawing, but is not going to fix a problem in my opinion, because there were never one, and is going to change the feeling of the work.
Don't get me wrong, If your drawing were bad, I'm the first to tell you "you need to learn this or that..." but for this piece and style I think you are in a good place. Don't stop creating!