Integrating large PostgreSQL table as partition for a new table
Proceed like this:

Don't create a primary key constraint on the partitioned table for now.
Rather, create unique indexes on the new partitions that will later become partitions of the primary key index.

As soon as the large table is old enough that you don't need the data any more, drop it.

Then you can create a primary key on the partitioned table that will use the already existing unique indexes as partitions.