Congrats. Give yourself a pat on the back.
So when you use the Copy Master detail it will copy every field, like it or not.
So what I used to do do when I have fields to clear out or fields to change, like say a [Status] field would be to have a formula URL field which would detect that the record name now begins with "Copy of", and and make the users push a big red button for Copy Step 2. That would clear certain fields.
Another probably better option now with less code is to use an Automation to do this. Especially if you are not landing the user immediately on the Parent record you just created, then the Automation will have the split second it needs to run and do the updates before the user sees the record.