To piggyback on the answers above, there's one simple addition that will help you:
An actual trigger warning.
This is, quite literally, what trigger warnings are for.
So that people who've experienced specific traumas can approach those traumas with time and consideration, gradually, and purposefully, exposing themself to the subject matter in a way that allows them to deal with the issue, instead of having it simply sprung on them in a way that might not be helpful.
There's a strange misconception that trigger warnings are about cocooning weak people from a harsh world. Nothing could be further from the truth. A trigger warning is about helping people who've already overcome some kind of struggle, to become even more resilient.
A trigger warning is like a splint. When someone breaks your leg, another person coming along and telling you to walk off a broken leg isn't really helpful. Someone calling you weak for not particularly wanting to walk on a broken leg is not helpful. Someone, metaphorically, kicking the crutch out from under you, when you're not expecting it, is also not helpful.
A trigger warning is an invitation. It's going up to the person with a broken leg and saying: "Hey, I'd like to help you with your injury. Do you mind if we cut the cast? Do you want to try a few stairs? If you aren't ready, it's cool, you can always stop the exercise and try again later. I actually respect you enough to bother asking."
It's as simple as a sentence that reads something like: The author would like [their] readers to know that this work of fiction explores issues that include of depression and suicide."
That's it. You could even go further and add a nice note, following up the above with "To anyone who's struggled with those challenges, I dedicate this book to..."
This isn't a hurdle. It's obviously something you care about, and expressing that to the readers deepens your investment in the act of creation and helps build their admiration and trust.
On top of that, as people have already commented: ask people who've experienced depression. This character is like any other character. If your character were a biologist, you'd have to research that. You'd need to know what sort of people are drawn to that, what sort of skills they need, what sort of baseline philosophies of the world does that attract or induce, what sort of tasks they habitually do, how that impacts their personal life and professional development.
If you want specific advice? Depression works differently for different people. Some people are high functioning and manage to drag themselves out of bed, even though their body and mind are screaming at them, constantly, that there's some aching, omnipresent, pain.
They smile. They work. They fuck. And the entire time there's a great, gnawing emptiness that makes them want to do, literally anything, to end the pain of simply existing
There are people who cannot get out of bed. They lose their jobs and friends. They try to correct the pain with anything that boosts dopamine: drugs, sex, escapism, whatever. Their life slowly crumbles.
And there's a groaning mass of people in the middle, who function...barely. Who hold down a job but have perfomance issues, or feel it spike when there's a specific trigger.
There are people (ahem) who are so detached from their own internal emotional landscape that they don't even see the world collapsing around them. They don't know why things are getting worse. They don't know why people are angry at them. They don't know that they are self-sabotaging. They just get less and less functional, and only actually realize that they are depressed when they finally break.
To understand the myriad faces of depression requires the exact same work as understanding any facet of any character. You don't necessarily need to ask people who've experienced that, or are experiencing that, really personal questions, but you do need to invest the same research you'd need to for any other character.
That's when your character will breathe.
That's when you'll understand who the trigger warning is helpful for, and who it isn't.
That's when that nice foreword becomes something real, and heartfelt, and a genuine connection with a people who can empathize with your MC because they see themselves in him/her/them.
As with most writing issues, you're thinking about it too hard.
Warn the reader.
Pour your heart and skill into the art, and watch the work become alive.