And I used all my rental mailbox money over the last 9 years since I first started investing to buy more and more.

If you make good money doing that, do it again a year or two later. 

I know making more money doesn’t make me right.

Yes, you can do DSCR to get your money back.

Given their age and position in life… mailbox money flow forever sounds like a good start to retirement to me!

Quote from @Scott Tennell: For real estate investors who have worked with private money lenders, what strategies have you found most effective in structuring deals to ensure both parties are protected and the funding process runs smoothly? 

We then use our money for the improvements.

Your name and credit would stay with the existing note, which you would keep paying on - ideally yourself with money you receive from the "buyer", because if they don't pay your credit is wrecked and your house is in foreclosure.If you search on here you'll see everyone and their brother looking to do Subto and it's a nightmare unless the buyer has the funds to pay the note off if/when the original lender figures out you don't occupy the property any more and are attempting to do what you want to do, since most loans are going to specify that performance.  

Consider financing options like hard money loans, or FHA loans for house hacking, and focus on areas with strong rental demand near the MA/NH border.

I'd need to finance with hard money.