This is how i do it. I finish the site in my own wix account. Sometimes i even upgrade to premium and include the expense in my invoice. In the final launch after client has reviewed everything and approved, i transfer the site to client’s account when he has cleared my outstanding amount for the design.
OR
you could complete the site in your account, transfer to client’s account when he clears your bill. Then the client could just upgrade to premium by himself through his account.