I don't think you will get as many answers from people who use c3d professionally. Not that they don't exist, but some of them don't comment here very much. Another thing is that at least some use Cheetah for rather a narrow kind of work, like modeling and texturing only and later use the model in unreal or so. One of the professionals has some pictures in the gallery here. He creates Characters and images for children's books, for which Cheetah is more than enough.
Second: some people do use different software, one for this, one for that. What can be done in Cheetah is done here, some other stuff in other software suited better for that. So while they earn money with Cheetah, they wouldn't without the other software they have in use.
Third: Cheetah is good software, very stable, in some parts really a work of genius. I admire Martin for this. In the end, though, Cheetah isn't really aimed at the professional market. Too much "can't do". For example the lack of SSS means you can forget rendering some things realistically, which shouldn't be a (big) problem in Archviz.
Fourth: Some professionals point out that they can't use blender professionally without paying for additional plug-ins which sometimes can't be used anymore in the next blender update.
Sixth: It's actually not important if others make a buck or two with Cheetah. It's the question if you can in your intended line of work. That's important in several ways.
Let's be honest, you can't compete with the top archviz professionals. In another forum some complained (I don't even know if they belong to that top) that the field changed very much for them in the last few years and they don't have any projects anymore that don't need billions (!) of polygons. Obviously such things are out of reach for you.
You can't compete with some of the cheaper firms who deliver shitty quality but cheap (they let work some underpaid 3d professionals in India or so for them).
So you have to find your niche, where you certainly produce better quality than that which comes out of any CAD-Solution. But you can be cheaper than the top notch firms. Is there a market? I think so, but I don't know that much about that field, so my opinion is to take with a grain of salt. Can you do it with C3d? Maybe. One of Cheetah's shortcomings is: no fur, meaning in your case: no easy grass.
One of the specialized archviz-renderers is Lumion. There you just load the CAD-Model (or 3d model from other software), use some materials, props and so on and render. The quality is really, really high. See:
Lumion in your language.
Do you have a chance to get into archviz? It's possible. You already know someone who maybe is willing to help you and maybe can introduce you to some architects. Look at websites from architects, and if you can reach the same level of quality, they may be your future customers.
For the first step at least Cheetah suffices: Learning.
I think that in this field Cheetah shouldn't be bad, and in the end, you could come up with some renders that really shine (you have to wait forever till it's rendered, though). But first you have to reach that level yourself, and then look for the software you really need. This could be Cheetah, maybe Blender or Lumion, who knows, probably rather a combination of different tools. But I believe that Cheetah can be better than what we have seen here until now (for me some shortcomings just put it out of competition, but I'm not into archviz).
But whatever you do, you have to be competent in your field, meaning you have to understand it, be able to model fast and learn as much about it as you can, meaning not just the technical or artistic aspects but as much as you can about architecture itself. A book or two will help. So an architect can talk to you in his language.
(and sometimes it's astonishing with what people can get away with and get paid for it because they know how to sell their stuff. Something I'm rather not that good).