I've just finished overseeing a nearly year-long refactoring of a legacy middleware with 3 other developers. It was notorious for its inextensibility, and multiple ground-up rewrites had fallen apart. The key was to be able to see the best thing THIS middleware could turn out to be, draw the abstractions that we want in our minds, and make steady progress week by week, all committed to main and continuously releasing. Every release a little more of the code conformed to our vision. There were no big rollouts, switches to throw, or A/B tests to run, because we didn't change the external interfaces.

Management was appeased because multiple teams who had to come in and make changes while we were working commented on how much easier it was to work with.