You make the payments to the lender.Oh, and just in case, there are more items to know about when considering a "sub to" transaction but this is just a comment on the payment aspect. 

It’s ran like a business, not a personal investment.Given the heavy workload I have being the CEO/founder of the wealth management company, my wife has set up a well oiled machine on the property management side, due to her perfectionist drive however, it still means that she's constantly dealing with tenants and toilets and replying to items at all hours of the day and night.

This is just one item. 

Without knowing any of your financial position of your work or side hustles it is hard to give you exact advice because you may be able to carry these homes until they are paid off and in that case they would provide solid returns, but there are likely better real estate investments out there.  

Many years ago when foreclosures were the hot item and every idiot in town was getting rich from them till the bubble burst, I paid $180 for Carlton Sheets No money down deal course hardcopy books and cd's well I bought the house I was renting with 0 down.

At the end i will add a list of items why adding a single ADU in single family zoned areas may not be the best RE option.The most common value add is via a rehab.  

Ironically the invested capital is often very similar for either. because your either investing capital in # of locations to scale, or investing capital in higher quality ie higher cost assets and operational items. 

If you are on cash accounting, yes you could do that.On the other hand, you are ignoring carry-forward losses. 

I would try to buy properties starting at a specific date so not large ticket items come up.

Having good accountants will be an important item to afford scalability.