Fill factor for later bigserial + index
Assuming you are using COPY to populate the table I don't see a reason to create the table without the bigserial column. 

Just create the table including the bigserial, but don't specify it in the COPY command. It will then automatically be filled with values during the import.

create table foobar (id bigserial, col1 text, col2 text);
copy foobar (col1, col2) 
   from 'foobar.txt'
   with (format csv, header false, delimiter ',');


That would remove steps 3,4 and 5.

I'm not sure what you mean with "range of bigserial" in step 6.