You don't know what the situation is.
It's in your best interest to take a step back and really try to be in your coworker's shoes. There's many unknowns that explain everything.
Despite your frustration, it's still not professional. You don't know what's going on. If you cannot empathize you're unqualified to be a software engineer and should pack up your stuff. It's not about you or a silly project everyone will forget about in a few years. Your coworker may be making up excuses because they don't want to share what's really going on with the rest of the team, especially if the other members are poking around. A close family member might be sick for all you know and management has enough heart to let their performance slide.
"It's come to my attention that x has only committed 8 times in the past ~2 years. I'm not aware of other substantial work that they've accomplished. Should I be slowing things down? I love this company and feel like my accomplishments speak for themselves, but I don't want to risk burnout if I'm working so far beyond the level that is actually expected. My understanding is that this concern is shared by my peers."
Who is assigning tasks to this person? Do they not notice that they aren’t being completed? I assume you and your other co-workers aren’t covering for them by doing their work as well as your own. If your work is being delayed because you need their work, let your management know. If this is stressing you, ask for a reassignment and give your reasons.
Concentrate on how this affects your work, specifically, and take that to your manager. Then let it go.
https://www.askamanager.org/2014/10/how-to-deal-with-a-lazy-...
You have to complain more. Complain to the boss's boss if your boss doesn't want to do anything. My experience is most people don't want conflict so nothing is done for the first couple of complaints.
Look, I just have a lot going on right now, Tucker. It's not easy being on HN and at the helm of a large project.
If you have already informed your manager of your observations and your feelings, then you have already done all that is necessary, because your coworker's level of performance is not your responsibility.
> This person's apathy and laziness is really starting to affect my ability to do good work.
This is a problem you actually have control over, so this is what you should probably focus on. Your coworker's behavior is not directly harming your productivity, after all; your feelings about the situation are doing that. If you want to keep this job, as it sounds like you do, you may need to accept that your underperforming coworker is a fact of life, then figure out how to get over it - you cannot control what, if anything, management will choose to do about the situation.