A rental with great cash flow will also help but its a pay off gradually rather than a quick pay off/down.Cash out refinance is a better way to get the funds usually because the cash out refinance offers a lower payment. 

In the case of the HELOC, you use it like a checking account so all of your surplus funds pay the balance down gradually and again, you're technically making extra principal payments every month, but since you aren't really missing the money it's like your mortgage is automatically being paid down faster.

If I don't evict any tenants, and they make payments on time, then I'll have to increase rents gradually each year, which will reduce the immediate value.So am I being overly optimistic in my thought process?

Though, I'd do it gradually especially if you have long term tenants living there.

One in particular was paying $550/mo we bought them a new stove & upgraded the furnace as their heating costs were very high.We raised it to $820/mo & not a squawk they even offered to pay $1200 cash if we would add central air to the new HVAC so we took them up on it.Our 6-plex was gradually rehabbed & brought up to above mkt rents quickly with new tenants.

We closed up shop with the service business for good this past summer after gradually cutting down on hours put into it and I've been a full time Realtor for about 6 months now. 

I know if we stick on this path of gradually building our portfolios, we will definetly accomplish these goals of being financially free!

As I get a feel for the response rate, I'll gradually start adjusting both the volume and velocity.

Gradually work up to $8k at month end.

Like most cities in the US it mirrored immigration and gradually went from English to German to Irish to Italian, but when the 67 riots happened in Newark (the city next door), anyone who was white that could sell did so, at prices cheap enough that black families could afford to buy.