Unethical and even just shady people (agents or not) really get under my skin. 

Now I'm ready to make my next multi-family investment, but I'm struggling to figure out the best strategy if I want to grow my portfolio in a healthy and sustainable way that doesn't impede my work and personal life. 

Don't let it get under your skin.

Of course, it's not healthy to stay late like that so we don't plan to do this all the time.  

Other rules, like any skin in the game from the borrower?

Keeping that LLC "healthy" and legal requires a whole host of other activities - having an attorney draw up an operating agreement, getting an IRS EIN, setting up dedicated banking accounts, keeping good records, keeping the accounting separate from your personal, signing documents as the LLC owner/manager and not yourself etc.  

They insist you have skin in the game. 

He needs to have skin in the game or he'll never commit to buying the place.The 'standard' way of setting up a rent to own is like this:Term: 1-2 years.Price: Today's maximum market value + 5% per year.Rent: Today's maximum market rent + X monthly option consideration.

Prepare to build systems, be disciplined and make sure you have thick skin.  

They have more skin in the game then you do.Make sure you get clear title.