15-20% for STR and MTR here in Alabama.We have a STR on Dauphin Island and that PM charges 24% but a "lot" of routine maintenance is covered in that.

Goal is cashflow or at least covering a substantial portion of the mortgage. 

Can try to reposition to Class B, but neighborhood may impede these efforts.Vacancy Est: Historically 10%, but 15-20% should be used to also cover tenant nonpayment, eviction costs & damages.Tenant Pool: majority will have FICO scores of 560-620 (approaching 22% probability of default), many blemishes, but should have no evictions in last 2 years.

And not those that they bought pre covid but rather in the current market situations.

Can try to reposition to Class B, but neighborhood may impede these efforts.Vacancy Est: Historically 10%, but 15-20% should be used to also cover tenant nonpayment, eviction costs & damages.Tenant Pool: majority will have FICO scores of 560-620 (approaching 22% probability of default), many blemishes, but should have no evictions in last 2 years.

These properties cover my mortgage, allow me to save for maintenance/repairs, and are projected to be worth $7–9 million by retirement.

Since you won't be able to recognize it as a valid rental if you're charging under market rent and you also expect to be covering any house expenses that are outside of her budget, this seems the best choice. 

Does this exist in the commercial realm?

If you convert it to a rental and sell later, you may owe capital gains tax + depreciation recapture, unless using a 1031 exchange to defer taxes by reinvesting in multifamily.Leveraging $100K in equity via HELOC or cash-out refinance can help fund the next deal, but ensure rental cash flow covers debt, expenses, and reserves.

We just acquired an existing duplex in Fort Lauderdale and planning on tearing it down and build a new duplex instead.