However, the agreement to accept monthly payments may be specific to the owner himself.  

Anyone having major success with only buying SFH and renting them out to tenants to make good cash flow each month and re-investing?

Think of it as cash on cash return, you leave money in the house and in return you get monthly cashflow. 

I would recommend this rather than splitting the bill and trying to collect that amount each month. 

Bathroom will have corner entry shower insert, and a sizeable single sink vanity.2 zone mini splits will handle HVACElectrical will need to be redone 100amp wont cover both and due to the EV charger in garage, electrician says it will be pushing it with split 200 amp panels, possibly necessitating a split 400 amp service.

The UPDATED rental comps around the area shows $850/month/unit.

Those should be coming out in the next couple of months for review. 

For example, a $50 late fee once per month is another $600 for the year if they are late every month as you wrote.

Non QM, 8.125% interest rate, 2.625pts, 20% down payment, 12 month prepayment penalty period - 3.61% CoC2.

Heck, most contractors have to build in that profit to be able to pay their own monthly bills.