If I put myself in the shoes of the outside partner/investor friend, I would be very concerned because: 1) unclear and/or potentially conflicting business plan, 2) even with clarity in the BRRRR business plan, unclear in the viability of pulling it off with these market conditions, 3) the chances of dispute seems very high, especially if you're going to live in. 4 years ago, I went through a  similar thought process when purchasing an investing property with mainly my family money, while considering an outside investor to rehab for higher STR ROI... 

Determine the most likely sales price of the build (factor in the cost of the land also) and include all selling costs to arrive at your net profit and ROI. 

There are too many factors that you cant control .

Hopefully this helps clear up the main point of your post!

We had one where they wanted us to pay for the majority of the costs to replace 1 mile of sewer back to a larger main.  

Don't get me wrong:  cash flow is still an important factor, it keeps the lights on, it pays for repairs and upgrades.

ADUs can significantly enhance your property's value by adding living space that can be used for rental income or as additional living quarters for family members.The increase in value from an ADU can vary widely depending on several factors such as the size, quality, and functionality of the ADU, as well as its integration with the main property.

Maintenance costs are a huge factor in the profitability, or not of any investment property. 

I guess it's just too many factors to consider and it easy to get overwhelmed and cannot get accurate info and estimation when I conduct analysis (ex. property tax rate, insurance, maintenance fee, rent, repair, closing cost, etc.) 

Of course the goal should be on the first page by anytime but that is usually bad ROI since the price is the biggest factor on the rank.