I came across a basic error, but it's just a small trap for new players.
CREATE TABLE seq_test(a NUMBER) / ALTER TABLE seq_test MODIFY (a NUMBER DEFAULT sage_seq.NEXTVAL) / SQL Error: ORA-02262: ORA-2289 occurs while type-checking column default value expression *Cause: New column datatype causes type-checking error for existing column default value expression. *Action: Remove the default value expression or don't alter the column datatype.
Investigating ORA-02262 returns next to nothing. It's a red herring anyway because I didn't realise the ORA-2289 staring me in the face before I checked if my database had that sequence yet.
ORA-02289: sequence does not exist
Typical APEX table definitions will never be the same again, though an even more elegant solution is to use IDENTITY columns. See the performance benefits at oracle-base.com.