It's... complicated. I actually started a sort of "startup" (intrapreneurship/ startup within a corporation) to help designers make more money/ be more successful [1]. So I talked to many people and studied what makes them successful.

In my experience/ opinion, what sets a successful designer apart is... marketing, and client management. I mean, they're not bad at their trade - you have to be good enough; but the most appreciated ones were standing out to me not because of their amazing graphical skills, or innovative ideas - but on how structured they were in approaching the projects & the relationships with their customers. E.g. some tricks that I've learned and seemed to work well:

- get the client invested early: ask a token price to analyze the project - weeds-out non-serious clients who have no intention to pay; then get them to do some consistent "work" for you - e.g. one designer was asking clients/business owners to fill in a consistent form with details about his business, how he sees his clients, the market, the competition, what he does well (better than competition) etc. By the time the client finished this, (s)he was already invested both financially & with work/time, and actually _wanted_ the designer to succeed.
- Pay attention to what the client says; use the client's own words to justify design decisions/ present your work. Makes the client feel you truly understand him
- NEVER, EVER surprise the client. You don't want to proudly present you final work, hoping that he'll be pleasantly surprised by your skills; that seldom ends up well. You want to constantly keep him in touch with what you are doing, and ask him to make (inconsequential) decisions along the way. That way, he "owns" the work too, and you won't risk that he wants "just a small change" or to "try something slightly different" in the end.
- Along the same lines as with previous points - introduce "checkpoints" (with payments whenever possible) along the way, where you get explicit "ok"/ "you're on the right track". Once humans have agreed to something, it's psychologically hard for them to go back and ask for something else.

I don't know, there were lots of "tips" like that, but the bottom line is that you're a "small business owner" first, as a freelance; and only then you're a designer. So what really really matters is to learn to work well with your clients.

[1] The opportunity is real, I believe. I was afraid initially that people won't be willing to share their process/ how they work/ what makes them successful, but that turned out to be a complete non-issue. There's a sort of pride in all of the successful people, they don't feel they are successful because they know "a few secret tricks", and thus are in fact very open & willing to share details of how they work. It's sort of a way to brag about how good they are.

BUT - I got to a point where unfortunately I didn't have enough internal support in the corporation, and to make the project successful I would've needed to invest many nights & weekends of my own time, to build the MVP and prove its success. And all that, while retaining exactly zero ownership of it all - I would've basically worked for the hope of a bonus or promotion, and with zero control - even if I was successful, at any point a senior manager could've taken control of the project and taken it to a different direction that I didn't want. I just decided the rewards were not worth the effort, and gave up :(