Fantastic article! You have walked through the whole process in quite some detail. Many years ago, I did something similar, but the scale wasn't as big. But the difference was that we were not just doing a database upgrade on MySQL; we were also migrating one schema to another (as the app was moving from a monolith to a microservices-based architecture) added another level of complexity to the process. Although those were a tense couple of weeks, we were able to do it quite well by the end of it.
A fascinating read for anyone who loves working with databases at scale!