“and atleast get some dummy data into my site so I can build the custom
post types correctly.”
I would recommend creating the CPT first and adding at least one custom field AND dummy populate it manually e.g. price to the CPT as the WP All Import you need to specify the CPT first and when you get to custom fields they don’t seem to show up unless they have been used at least once (probably a WordPress thing)
Hope that saves you some grief!