You are thinking through this well. When you choose the launch on a secondary/ long-tail KW rather than the main KW, it might mean you have to do a series of ‘mini’ launches on several KW’s to optimize the breadth of your KW ranking.
As a next option, PPC is definitely an option for you too, but when you do a giveaway you ‘supercharge’ the ranking algorithm in a short space of time. So you rank quicker. While it might seem that PPC can give you a higher profit return initially, as you are making money on each sale(vs a giveaway), you are investing more heavily in PPC to pay for visibility.
So giveaway trumps PPC for speed to rank. Let me ask you a few extra Q;s
When did your giveaway end? – typically you need to wait 7 days AFTER the last day, for the algorithm to catch up to show you your ranking result.
Where are you ranked currently for your main KW? – i.e: Has the long-tail KW rank helped boost ranking on your main KW?
What ranking have you achieved on the long-tail KW
What are the average daily sessions you are getting at the moment – this will show you how broad your traffic is.
What is your average daily unit % conversion?
What is your current units per day (noting your target of 50 UPD)
How many giveways would you need to do on a closely linked KW to your main KW?