I think this is the wrong target. Large holders, because they have so much at stake, can be expected to react reasonably quickly during the second step you are describing. However migrating Bitcoin to PQ requires migrating the ~entirety of its current users to stop relying on EC crypto. So i think the target in the near future should be to provide a migration option to those users we expect to be the least reactive, such as we enter step 2 with as few users that haven’t opted into a migration as possible.