I've seen a bunch of successful promo packets for work on code health, system reliability, etc. - including promos to L6.
The "trick" to successful L5 and L6 promos (and above) for this sort of work is to have credible estimates of the actual impact of your work. Way too many engineers spend quarters or years refactoring or rewriting systems, and then go for promo with a case that's basically: "System X was kludgy, crufty, and engineers complained about it. I designed and lead implementation of a clean-up effort, and now people say it's nicer."
That's generally not going to cut it. You might get lucky, or you might get bailed out by a peer reviewer that provides solid data about the impact of your work, but you can't count on this, and you should be doing the legwork yourself.
You need impact estimates. That generally means you need some measurements, although the measurements don't have to be perfect. What metrics measure the pain that the existing system is causing? Some examples might be:
- Average # of hours required to push a release
- Average SWE-days/SWE-quarters/etc. required to develop a representative feature change
- Average monthly user-reported bugs
Spend some time actually measuring this stuff. Write some queries, run an internal survey of the developers on affected teams, etc. Take it seriously. Ideally, do all this before you've actually started work on the cleanup you want to do. Write a proposal doc, get it reviewed by others, and make sure they find your estimates credible. If the numbers are smaller than you expected, reconsider whether the clean-up is worth the time you're considering investing in it.
After you've completed your project, measure again. If your project is amenable to an experiment-style launch, that's ideal, but pre- and post- measurements are fine too. Share the stats - advertise your team's success! Package them up in a nice doc you can link to in your promo case.
"How am I supposed to find the time to do this while I'm doing my normal job?" This is your normal job. The opportunity cost of your time is $XXXk/quarter. The opportunity cost of your team's time is $X million-$XX million/quarter. The single most important thing you can do is make sure that time is being invested in a high value way. In your personal life, you would look for a lot of data before you invested that kind of money - you should be doing the same at work.
In general, as you move up you should expect to spend a higher percentage of your time figuring out what problems you're going to work on. As a result, the fraction of your time that you spend actually designing and implementing the solutions to those problems will decrease, but it's still a net win because you and your team will be focused on tackling the really important problems.
In summary, it's possible to be really successful and quickly move up the ladder by doing "sustaining work". The catch is that you have to be rigorous about choosing the specific "sustaining work" to spend your time on.