A few more comments.
First, you should be encouraged to try making characters to learn if you like this kind of modelling. It can be personally rewarding, and it pays well. Characters, especially unique and well-made ones that are rigged very well are the best selling products on the major brokering sites. But, this kind of model-building is its own specialty. Very different from prop modelling or VFX. Personally, I don't enjoy this, but I did invest about a year learning the basics, because a freelancer needs to know some of every kind of modelling. So, if it helps, here is some of what I learned.
Here are the skill sets you need to develop - in order.
Human body modelling --
... You'll need to become reasonably good at the polygon layouts for base meshes for male and female humans. This has everything to do with the flow of mesh lines and their relative densities, and it has to be based on a reasonable knowledge of human anatomy.
(an aside: to help explain why some study of human anatomy is important - are you aware that human nipples don't point straight ahead, for example? To be realistic, they point away at angles from the human body in the left and right directions. Or, for another example, suppose you want to model African or Asian facial types. Those differ in significant ways from Caucasian faces. But to get a good grasp of the ways in which they differ, you need to spend some significant time studying human facial anatomy. Anyhow, if you can spend some time on a tutorial or two about human anatomy and how it is modelled in 3d, you will come much more efficient in your modelling, and quickly able to achieve what you intend. Plus, it will make any sculpting significantly easier. And "efficiency" in modelling is CRITICAL - especially if you hope to earn some money from this enterprise.)
Next up - Rigging and Skinning
... These are two different skills. Rigging, that is, construction of bones - interior skeleton, is pretty straightforward. Just a basic skill, and any decent model-building program will have good tools to enable you to correct mistakes,
...Skinning is a whole different matter. Your tool of choice (Maya, or Blender, or whatever) is going to dictate how complicated and how nasty a subject this can be to master. I am a Maya modeller, and Maya probably has the most complex and sophisticated of the skin weights tool set, so I may be overstating how difficult skin weighting can be to learn. But, there are no shortcuts in this area. Skin weighting can only be learned through trial and error. It takes time and patience to earn a reasonable degree of competence.
Skin Painting -
This is another whole subject not mentioned above. It is its own skill. Really, you need to become proficient at Substance Painter, if you are not already, just to start. I might be able to save you a little time and money here. I recommend the tutorial by Andreas A. Martinez Romero on Artstation Marketplace. it is called "Skin Texturing Course (Heads and Body)." Of the various ones that I purchased during my learning period, this is the only one I thought really worth the cost. And then, of all the various tools that I purchased for skin painting, really, the only one I would recommend is "Human Skin Base Smart Material For Substance Painter" by Diego M. Zanelli. Also on Artstation Marketplace and you would want to pay for the commercial license. Actually, I don't use this much any more, but it was a great thing to start with as it sets up a good skin layer system in Substance Painter, and it enables you to start making decent skin tones for people of color quite quickly.
Hair:
I see that no one above is discussing this. This is also its own subject and specialty.
I have pissed away so much money on this before I concluded that purchasing all the "short cut tools" was a total waste. So, let me try to save you some time and money here. Things I learned the hard way ...
So, there are three kinds of hair technologies you can use - depending upon whether your characters are supposed to be real or stylized and whether or not the hair has to move realistically in animation.
If the hair is to be stylized for non-realistic characters, you don't need to know anything. Just purchase one of the very good hair basemesh kits on Artstation Marketplace and give that stuff some suitable coloring, or maybe buy one of the cheap hairstrand kits, and apply it.
If the hair has to be ultra realistic and move realistically during animation (or respond to wind, etc.), you are talking "Hair Gen - Grooming" technology, and you pretty much are going to need Maya. Skills here involve creating individual hair strands and braids, and grooming them into patterns and directions, and prepping them for realistic animation. (Not to mention coloring, which is another subject.)
In the middle ground, which is where most of the AAA character modelling occurs, we are talking about a technology that consists of making semi-curled planar strands for clumps of hair, arranging them in some good way on the head or a "skull cap" model that fits the character's head, and then making the hair strand images to be applied to those planar hair strands. There are various tutorials that demonstrate how to make the planar hair strands - specific to your model-building program. For some applications, such as Maya, there are specialty tools and plugins for this purpose. However, the critical key skill here really focuses on making the hair strand images. For this purpose, there is now only one tool that rules them all. This is one of the iterations of the "HairTG" tool by Oliver Lau. (By iterations, I mean there is one version for general hair, one for facial hair, one for feathers, etc.) Using this tool is a "skill" that will require time spent in learning (the basic HairTG product comes with tutorials) and practice. Just don't try to take one of the "shortcuts." Don't try to get away with any of the other cheaper hair strand generators or the purchasable hair cards. In the end, you'll be wasting your money and time. Hair making is a key component of character design and modelling, and you'll absolutely need to master Mr. Lau's tools in the end. This, now, is the only way to go.
Finally, we come to Clothing.
Here again, you need a completely different skill and tool not mentioned above - namely Marvelous Designer.
Characters are nothing without their clothing (and other props). So, I benefitted from first "Marvelous Designer Tutorial Starter Pack" by Travis Davis, that quickly taught me the basics, and enough to get by if I never learned another thing about making clothes in Marvelous Designer. The, I benefitted even more by "Tutorial - Becoming An Expert in Marvelous Designer" by Aleksandr Kirilenk. This latter is now outmoded by improvements that have taken place in Marvelous Designer, but it still covers all the important, semi-advanced subjects and it will give you an excellent overall command of Marvelous Designer. Once you've purchased a monthly subscription to Marvelous Designer ( you only need to purchase a month's worth when you want to use it. You don't need it all the time,) then you can purchase clothes for your character very cheaply (as in $1.50 to $16.00 USD) on CGTrader or other sites, and use them either as a template for your own clothing designs, or purchase the commercial license for any of these, just dress your characters and move on to the next problem. And by the way, by "Marvelous Designer Clothing", I mean typical clothing items, but also backpacks, shoes, bags of a thousand kinds, straps, buckles, fasteners, and may kinds of props you wouldn't necessarily think of as "clothes." ( I make boats and I use MD for sails.)
A final set of comments - just kind of related to some of the above comments.
There's really nothing wrong with starting with Mixamo, or even iClone and Character Creator. These are good applications that help you figure out if this is the kind of model-building you might want to get into. I make professional-level characters for some clients on demand, and, despite what was said above, I use Mixamo all the time for quickly testing out ideas for poses or animation. Nothing wrong with getting super-familiar with Mixamo and with the free ActorCore AccuRig for the same purpose. Even CharacterCreator is a good tool for some purposes. Just last month I completed a job for an artist making book illustrations that needed a 50 different ancient persians posed in some specific ways in a matter of one week's time. Fortunately, he used Max. I screwed around the first three days, making them with my existing array of Maya tools, before I figured out that Character Creator, in conjunction with my Maya and Marvelous Designer was the only way we were going to make his deadline. (Which we did do.)
So, take what has been said here, including my comments, all with grain of salt. Anyhow, hope this helps round out the answers to your questions a little bit. Nothing wrong with what you've done so far. Just know that character creation is its own branch of model-building, and that it will take some time, several distinct skill sets and patience to master. And that doing so if financially rewarding. I don't want to do it, I'm a VXF modeller at heart, but I'm glad I took the time to learn something about it.