I've never done anything close to that scale of migration, but I also can't really imagine taking an approach that feels like a huge migration.

I feel like I would try to strangle the old app (putting proxies in front and redirecting endpoints as they're rewritten) rather than swapping it out wholesale.

At that point I don't know if it really counts as a migration as much as just development.

Any thoughts on that approach? Is that what you have done, or was it a more holistic swap over?