Is galvanic isolation needed when the output voltage is higher than mains?
Yes, of course galvanic isolation is valuable. Regardless of the voltages being used on the secondary side, it prevents the user from being shocked by the mains voltage in the event of some sort of fault.

Now, given that you are probably providing lots of isolation between the high secondary voltage and the end user, and provided that this isolation meets the safety requirements for "double-insulated" equipment, then galvanic isolation between primary and secondary is not strictly required.