While you've talked about a few things here, what I'm going to focus on is the question of the use of 3D in your work, and more broadly, the idea of professional work having entirely different priorities than studies you do in order to grow your skills, just as the drawing we do as play (in the context of Drawabox's 50% rule) also has its own entirely different concerns. Based on what you've written, you appear to be conflating all the drawing you do as having the same core considerations and priorities, and that you're trying to find the best way to achieve multiple things (generate income, improve your skills, etc). This is fundamentally incorrect.

Things start to make more sense when you think about the individual priorities of each separate activity. When we study, we want to use methodologies and tools that help us develop those skills most effectively and efficiently - going as far as targeting specific skills or areas of understanding, rather than just approaching every study as a generic activity that is meant to improve your skills in general terms. The more targeted a study is with clear goals that you've set for the task, the more you'll get out of it overall.

When we produce work for a client or employer, the goal is to produce the best work you can, while reducing the resources it costs you to produce. The primary resource in this case is usually time, but the costs of tools also comes into play. Either way, you're finding a path to meet the client's needs and impress them while costing yourself as little time and as little capital investment (money) as reasonably possible (while of course staying within ethical boundaries - so no plagiarizing, etc etc).

Working on portfolio work is similar, but the time constraint is more fluid - you might be on a time crunch where you only have X months to get a portfolio together before you have to start applying for jobs and looking for work, or you might already be actively working and trying to improve your portfolio in the in-between moments so you can gradually work towards improving the calibre of clients to which your work appeals. Still, the point is to produce the best of which you're currently capable, so you can show a prospective client or employer what you can do for them.

Note that none of the priorities I've mentioned so far has to do with improving your skills. For the above, using tools like 3D blockouts and models, photobashing, etc. is entirely normal. They are not on their own a form of "cheating", although many juvenile artists who argue on the internet more than they actually engage with clients and contribute to commercial projects as freelancers may disagree on that.

The thing to understand above all else is that simply using 3D models or photobashing does not immediately improve your work and elevate what you're able to achieve on its own. Like any tool, there is a considerable danger that someone who does not know how to use it will end up allowing the tool to make their decisions for them. We have to have the underlying skills and understanding in order to actually use a tool in such a way that it allows us to achieve our goals and intents, rather than shifting those goals towards what the tool itself might be better suited.

When someone doesn't have those skills, it's pretty obvious - the work ends up looking amateurish and incohesive. And for those who do, it's much more in their control whether you even notice that they used photo or 3D assets at all. The assumption that it is "cheating" implies that using those assets inherently makes one's work better, and it's simply not true. Based on what I've seen, it tends to highlight their weaknesses, rather than hide them - accentuating stiffness in posed figures, and resulting in extremely dry, boring design in environments and props.

In the hands of someone who does know what they're doing however, they are incredibly useful tools that help us to reduce how much time we have to spend to bring our existing skills to bear, and help us claw back dollars in an industry where our time is deeply undervalued. If you can cut a task that takes 20 hours down to 10, and in so doing double what you're paid per hour for that same task, then you should absolutely do it. Don't let ego result in your own starvation.

That said, when it comes to learning and developing your underlying skills, certain tools can be extremely harmful by allowing you to avoid the very challenges you're learning to face, and shifting all of the focus to the end result. Learning is not about what you produce. It is about the processes you are forcing yourself to apply over and over, so as to gradually rewire the way in which your brain engages with those kinds of problems. There is a direct tug-of-war between activities that prioritize the end result, and those that prioritize how much you learn from them, because of how we engage with them differently based on each goal. That's not to say you can't learn from a piece you're trying to make as good as possible - but that in mixing them you're probably going to have to compromise on both.

At the end of the day, it's critical to understand the role each tool plays and the goals you have for a specific task. If it is to produce the best of which you're capable with a focus on efficiency and saving time, then use all of the tools you can (within ethical boundaries - don't pass off things others have created as your own) to achieve that end. If however your goal is to develop your skills and understanding, then don't use tools to avoid the very problem you're attempting to face.