question on audit columns

Hello,
In postgres database , we have all the tables with audit columns like
created_by_user, created_timestamp,updated_by_user, updated_timestamp. So
we have these fields that were supposed to be populated by the time at
which the insert/update operation happened on the database but not at the
application level. So we are planning to populate the created_by_user,
created_timestamp columns by setting a default value of "current_timestamp"
and "current_user" for the two columns, but no such this is available to
populate while we do the update of the row, so the only option seems to be
through a trigger.

So wanted to check with the experts here ,considering the table will be
DML heavy table (300M+ transactions will be inserted daily), Is is okay to
have the trigger for this table for populating all the audit columns or
should we keep default for created_by_user, created_timestamp and just
trigger for the update related two audit column? Basically wanted to see,
if the default value does the same thing as a trigger or it does something
more optimally than trigger?

Regards
Yudhi