Adding primary key on table with 3 billion records.

We've been trying to figure this out but has no luck. Hopefully someone there would have an answer.

What we've done so far:

1. We make sure the ID column has no null value

2. We created a unique index on the ID column concurrently to avoid locking

What we are planning on doing next

1. Add a not null constraint on the ID column with NOT VALID option so that it will run very quickly

2. Run the validate constraint check. It takes a really long time but at least there's no locking

3. Run the "alter table add primary key with index" command to promote the index into primary key.

Here are some issues we're having right now 

1. The DB started autovacuum after we created a unique index. It's been running for a really really long time and it won't let us alter the table to add null constraint. I tried to cancel the autovacuum job but it just starts backup.

2. I created a duplicated database and was able to get to add primary key with index step. However, it seems like even though I've already validated not null constraint, the alter table command still took a really long time to run. This alter table command has a read/write lock, which is problematic for us on a production database.

My questions are:

1. Can we safely cancel autovacuum job multiple times? I've canceled the job twice without knowing that it'll come back up. What's the consequence of doing that?

2. Is there anyway to make the alter table add primary key step faster? I'm not super familiar with this but from I've read in the documentation, nullable constraint is the only difference between primary key and a unique index. If I've verify in the pg_constraint table that the not null constraint has convalidated column = True, why does it still take so long to add the primary key?

My postgres version is 9.5.15 and we're running this on AWS.  Really appreciate any help in advance!!!

CREATE UNIQUE INDEX CONCURRENTLY table_pkey ON table USING btree (id);
ALTER TABLE table ADD CONSTRAINT table_pkey CHECK (id IS NOT NULL) NOT VALID;
ALTER TABLE table VALIDATE CONSTRAINT table_pkey;
ALTER TABLE table ADD PRIMARY KEY USING INDEX table_pkey;