In Oracle 12c trying to create table with columns greater than 4000
Those are not the correct steps to enable the extended data type. You have set max_string_size to EXTENDED in the spfile (scope=spfile) so it had no effect while running the scripts.

Try again.

Non-CDB:

Enabling the New Extended Data Type Capability

CONNNECT SYS / AS SYSDBA
SHUTDOWN IMMEDIATE;
STARTUP UPGRADE;
ALTER SYSTEM SET max_string_size=extended;
START $ORACLE_HOME/rdbms/admin/utl32k.sql
SHUTDOWN IMMEDIATE;
STARTUP;


Or:

MAX_STRING_SIZE