> The same people leave detailed comments on others' merge requests, but get discouraged when nobody else puts in the same amount of effort for theirs.

This is always a precarious situation. Because as soon as these people become jaded, your ability to make good PR culture will also vanish. And they can become jaded for many reasons. If these people are not explicitly or implicitly valued, they will know. If people who are doing the incorrect things are getting promoted first (or even at the same rate!), the same raises/bonuses, and on all accounts are treated equally, the employee will almost always converge to "well why am I putting in all this extra hard work if it's not benefiting me in any way?" And I don't think promises of early promotion or similar have a good effect because there's many employees who've had those promises made to them and it not follow through[0]. So there needs to be some, even if incredibly minor reward in the shorter term.

Also, do not underestimate the value of explicitly saying "good job." There's often a huge bias in communication where it is only made when something is wrong and when good work is done that it is left unsaid. You don't have to say it for everything, but I think you'll be surprised by how many people have never heard this from management.

[0] I wanted to share a story of an instance I had with this. I was a green (mechanical) engineer working at a startup. I had a physics degree instead of a ME, but have always been hands on. But because of this I was paid less and not valued as much. I asked my manager what I would need to do to get promoted and to be on par with everyone else. I got it in writing so I could refer back to it. At my next performance review I was just talked down to. Complaining about how I didn't do this or that (sometimes things that were impossible and sometimes they were weird like "your code may have been 20% faster but X couldn't understand it so we can't use it" -- X was a manager who had only been writing in C++ for < a year and I __heavily__ documented my use of functors). I asked about the things I did and the promises. They admitted I did all of them and even more. One of these being getting a contract (I think they put that there not expecting me to get it), and I was the only non-manager with one, bringing in 20% of company revenue while being the only person on that project. You can imagine I walked out of that meeting polishing up my resume and I was strictly a 9-to-5er doing the bare minimum from that point on. But the next manager I had, was liberal with complements and would critique instead of complain. Understood that there were unknown unknowns and all that and would actually tell me to go home when I was putting in overtime. I never worked harder in my life AND it was the happiest I had been. A manager can make or break an employee. And to part of this is that there may be ways to get back those broken employees, but you might need to figure out why they became broken in the first place. And if it is something you can fix or not. I believe environment has a big impact on employee attitudes and thus, efficiency/productivity. If passion is worth 10 IQ points, then happiness is at least a big factor in making an employee productive. Everyone can win because it isn't a zero sum game.