I’m not 100% sure if this is what you are asking, but if you put 10% down at purchase you probably still have about $360k on your mortgage.

Promissory notes in general, as they have no collateral, seem a bit higher risk, perhaps mortgage note investing which are backed by residential and sometimes even commercial real estate would be a safer alternative, and yet very lucrative, too,  8-12% yieldsFor extra returns, I put a small portion of my portfolio into BDCs, Business development companies, that specialize in loaning money to growing businesses which are too high risk to get loans from proper banks.

Although we’re not a local bank or credit union, we are independent mortgage brokers with experience in financing options that consider rental income rather than personal income. 

I've been playing with amortization calculators and have seen you can pay off 1/3 of the mortgage and reduce 2/3 of the duration.

I have just over $200K left of owner-financing, but I'd like to obtain a cash-out mortgage/loan of $500K (ideally) to be able to pay off the previous owner and make some repairs, etc. 

.: I'm thinking of investing in some notes with Capella Mortgage.

Mortgage is only less than $1400.

I was shown about a handful of their holdings in the Houston, Memphis, and Little Rock markets, but the cash flow and CoC returns were less than $100 per month and 2-4% after factoring in additional expenses like 7% vacancy and 10% maintenance per month to go along with 20% down and conventional mortgage.

minimizing free cash flow with larger mortgages or shorter term mortgages takes the cash flow which would be taxed and puts it into the amortization of the loan. 

., Honesty is the most important aspect in mortgage lending.