The cost of the loan is slightly higher than a conventional loan but not by much. 

This strategy has allowed me to save on closing costs, secure favorable financing terms, and refinance almost immediately, all while minimizing my down payment.

Bull***t, that only white people can reach while everyone else languishes at lower altitudes.

I am VERY certain I have made some (or many) mistakes or left money on the table or overpaid etc... but all of these deals are cash flow positive after all costs + vacancy and maintenance. 

Instead, I should:Record the net rent (gross rent minus PM fees and expenses) as income in QBO.Log any additional expenses I directly pay, such as mortgage payments, insurance, property taxes, and other business-related costs like gas for business travel, meals, business licenses, and office supplies.Does this approach align with the correct practice of differentiating property accounting from corporate accounting?

Understand the fees involved and calculate the total cost for an entire year of management so you can compare the different managers.

In the past, it was a pretty low risk especially if you manage the property correctly, take certain precautions and even certain property types.  

Repair & installation costs in the Boston area are some of the highest in the country.- Watch out for residential tax exemptions, and be aware of which towns / cities have them.

'Cashout refi' will confuse your lender and cost you more.

Sure, but I agree that until you really know your problems with scale you can't justify the cost of a custom or premium software solution.