million in addition to their investors (who expect a reasonable rate of return) by keeping the rental rates at a fraction of the market rate?

I believe that you don't get returns (failed to deliver) with bulk mailers so you don't know which addresses are bad.That's the simple stuff.My advice is to do various mailings that you or some prepares locally, that go out immediately, using 1st class postage.

You get a Capitalization Rate (CAP Rate), which equals your annualized return by dividing the Net Operating Income (you had gross in your narrative) by the purchase price.

This implies your return numbers are inflated.  

We can even return deposits through them too. 

Even if a title company were to manage the setup making sure all parties are paid, the cheap debt and cash flow would not be enough to be enticing to any investor even if midterm rental could yield a healthy return. 

Many of which are savvy BP type clients who are doing updates/raising rents and then the buildings do cashflow positive even with lower down payments which produce amazing returns (at 3.5-5% down you get close to a 50%+ return a year just off mortgage paydown). 

Leverage magnifies return. 3) The effort involved in adding an ADU is comparable or larger than a rehab associated with a BRRRR.

This is not a great return, but not terrible.As others noted, there are other risks, like having the property insured at full replacement or purchase price, or if the option is 100% seller financing with no money down or very little, versus 20% traditional down payment, this can start to move it into a better investment.

Quote from @Jed Butikofer: Unfortunately, it takes money to make money.With high prices and high mortgage rates, cash flow is difficult to find and we may not see a return to the 2018 market for a while.