I also think you have a misunderstanding of amortization when you question why you pay so much interest initially.  

Also, providing value to others at a high level will pay off for you in the end if you stick with it.

To pay for this, I used a heloc on my current home.

And particularly one that pays but a fraction of their real job. 

Why pay off the mortgage early? 

If not, who do you believe the natural buyer is and why do you believe they will pay retail when they can replicate what you accomplished with any number of available properties on the market at any given time?

You don't provide enough data to know.Every product is only worth what you are willing to pay for it. 

The perceived value may be more, because you know an STR was run there and making money, but a lender wont value it more.While I agree a lender probably won’t value it more, an investor interested in running it as a STR will pay more.  

You don’t have to pay $20k+ to learn to broker loans. 

They should be found early on by talking to other investors or your investor friendly Realtor.It is through effective property management that your money is made besides buying well initially.