When it comes to making deals, the more you know about banking, the better off you are

I believe that is a better option because most banks won't pull a HELOC off an investment property. 

If we go off of an average of 35% expenses (I use 35% for properties older than 10 years and 25% for properties 10 years and newer), then we sold that duplex at a 4.75% cap rate.  

They had the money, the will and the marketing genius to pull it off.

You could start BRRRRing houses and maybe that would allow you to buy more and make more cash flow.As for the paying off the mortgages, I would make you minimum payments monthly and when rates go down in years, you could refinance but pay the same amount and that will help you to pay it off sooner while you are left in the same Cash flow position. 

However, it may turn people off to nice investments that can give you anywhere between 12-15% ROI (singles, doubles, and triples.I'm far from an expert and I want to learn different ways you guys analyze deals.

I can't afford to pay off the 500K myself and either can the tenant for a downpayment. 

Also as for your comment regarding pay off the existing mortgage in 2-3 years.

I think most of the ones I have come across are either a) Very well off, and diversifying, or b) much later in their careers and they are using a vehicle they know (RE), just in a more passive way. 

Meaning will it wipe out the majority of your write-offs moving forward?