So much of the answer, for better or worse, depends on company culture, personalities, leadership, etc. But... To the extent possible, prioritize complete vert

So much of the answer, for better or worse, depends on company culture, personalities, leadership, etc. But... To the extent possible, prioritize complete vertical slices. While it may not be feasible for foundational or generic technology layers that must be fully developed before building on them, aim to avoid focusing on large horizontal layers that delay end-to-end functionality or demonstrable results within the new system. Again, depending on many factors, you might consider the strangler fig pattern, in which you implement some component of the new system and operate alongside the original one, route some usage from the old to the new (or even run in shadow), test and validate, repeat incrementally, and so on. In my mind, what it comes down to is just being really vigilant in ensuring you minimize the risk inherent in a massive migration...which basically means try to achieve the ultimate migration with a series of much smaller, iterative migrations that carry significantly less risk.