In a mortgage assumption one borrower takes over another’s existing home loan.

I'm not sure how much you are borrowing for the purchase and rehab but say you refinanced at 75% - that would be $90k which is greater than the purchase price + rehab ($85k).

-Borrowing money will likely just continue to become more difficult for the foreseeable future (the terms you'll get in 6 or 12 months will probably be a lot worse than the terms you get today); so you can't really depend on future refis (or, at least, you have to be very conservative with your expectations for future refis).

I won't be able to borrow anymore money after these three close without using hard money and those fees are high. 

Getting that mindset right, at any age, allows someone to grow beyond their own means, and it has been my observation that is true of literally EVERY successful investor - growth has to be beyond their own means….even the millionaire needs other millionaires to fund billion dollar projects….so at some point, learn to depend on others, partner with others, borrow from others and this will greatly expedite the journey.  

So if you make a $50,000 annual premium, you should have something more than $40,000 that you can immediately borrow against.

Interest rates in these spaces range on what kind of FICO score a borrower has, where it is located, and a variety of other factors. 

It's based on the borrower's credit and the property's income and that's it. 

Is a  non occupant  co borrower  ( like a  relative/ parent ) an option ?

Why borrow cash if you do not need to?