They’re also a junior engineer (less than two years), so I’m still not sure if I should be looking to push them out.
My biggest uncertainty is what is an objective criteria and expectation for a junior engineer.
The fault is mine.
Earlier on in my career I would read stories from tech companies that said something like "if you don't get promoted to a certain level within 2 years then you get fired." Initially I thought that was pretty harsh. Now, I understand where it comes from. If someone hasn't shown progress in 2 years, assuming proper feedback, it's doubtful that they'll ever improve much.
On the other hand, I believe there are places for average engineers. Part of the issue with this person is that they wanted to keep trying to do more complex work even though they struggled with easier tasks and it was a constant struggle to try to balance that.
I think in retrospect, had I been the person's manager, the best option would have been to restrict their work to the easy tasks. If they didn't like it then they could always leave. I now also understand why some people are "managed out". Maybe they can stay in their role but if they don't have the ability to progress then I don't think it's a manager's duty to help them beyond a certain point.
It's also possible for and employee to get stuck in a rut at a job, and nothing can get them out of it except a change of scenery. This is because everyone's expectations of them being a low performer or troublemaker or whatever are already set, and that perception will not change. So it ends up becoming a self fulfilling prophecy, because any small improvements the employee makes don't move the needle on how people perceive them.
Keep a close eye on how their underperformance is affecting other team members. If the rest of the team has to make up for the person's underperformance or rewrite their code, they could be subtracting rather than adding value to the team. Engineers are rarely islands of productivity.
Consider your headcount. If you can only hire 5 people and 1 of them isn't really contributing, you're giving up a full 20% of your team's potential. Replacing that one person with a performer on par with the other 4 people on the team will instantly boost their performance by 25%. Withholding that kind of advantage from your team isn't good.