Opportunity cost of switching to a conventional w/ private lender

[Bret Kingcade]

This week I went under contract on a duplex that I planned to finance with an FHA loan at 3.5% down. Right after I told my family about the deal, my dad reached out to me by surprise and told me he'd be willing to lend me the money to enable me to use a conventional loan. His suggestion would mean giving me the remaining 16.5% of a 20% down payment and that I would repay him over 4-5 years with only 2% interest. This is no doubt a generous offer for him to make but the more I think about it, the more I wonder if it would actually be the right financial decision for me. Please forgive me for the novel but below are the relevant facts:

-My lender has told me that this would not cause any issues if I made a decision to change this by Monday as far as the closing process goes.

-This is my first rental property and my goal is to progressively scale as quickly as possible within reason and to hold my properties long term.

-The deal already works out acceptably as an FHA loan with 3.5% down. If I switched to a 20% down conventional loan, cashflow would increase by roughly $200/month (before taking into account repaying my dad).

-If I borrowed his 16.5% down on a $136k purchase price and I paid him back at 2% interest over 4 years, that would cost me $487/month. Over 5 years would be $393/month.

So basically my concern is that for those first 4 to 5 years, I would be redirecting all my cash flow plus some disposable income in order to repay my dad. This would noticeably slow down my ability to save up for a down payment on my next property which means there would be an opportunity cost to accepting his offer. I am relatively young at 27 and want to use my available time to my advantage. I am having a hard time making a decision on this one. I really appreciate people sticking around and reading through all this. Any input on this would be very valuable to me.