1) DB2 is quite different from IDMS.
2) You will need a good, solid basis in RDBMS theory.
You will need in depth DB2 knowledge.
You will need in depth z/OS (MVS) knowlege.
Have you ever used SQL? If you are going to be a DBA you need to know what the SQL does “under the covers.”
I good DBA is a blend of DB2 expert, systems programmer (because you will be installing DB2), systems analyst (because you will be working with the applications people), and a you need to be a drunk (so that you can sleep at night).
3) yes. ANYTHING on the mainframe requires JCL. And as a DBA you will be expected to be an expert in EVERYTHING.
4) I don’t know.
Book learning is a start.
SQL is SQL, but how it is used by DB2 is different than how it is used by Oracle or by SQL Slower. It is even different between DB2 on LUW and DB2 on z.
One does not start out at the top.
Maybe get a job as a DBA trainee to start.
Nothing can educate a person as well as hands on working with someone who knows.