50M rows is really not that much, I’d guesstimate an index creation to take single-digit minutes.

None of these operations I’d expect to cause downtime, or require table duplication or to be risky

Edit: to be fair, you’re right there’s footguns. Make sure index creation is concurrently, and be careful with column default that might take a lock. It’s easy to do the right thing and have no problem, but also to do the wrong thing and have downtime