Was having a fun, purely academic conversation with someone and somehow the topic went from the stock market, ETF's, to real estate, to BRRR's and either he or I or both us came up with this and wondered is this a legit strategy that others have used?

That looks like your problem - only 3% down (can you get that conventionally if its a pure rental property - i'm not 100% but I don't think so) - with that leverage (97% LTV), pretty much nothing is going to pencil.

In other words, how would I calculate the return (from a purely financial standpoint) of "investing" in a primary residence vs renting and putting the money elsewhere (perhaps even into an investment property, but also considering buying a business)? 

This is typically more favorable than financing for purely investment properties.Income Generation: You can rent out the other half of the duplex.

They're designed for owner occupants but could be something in which you occupied for a duration of time and then converted to a pure investment later. 

The problem for me is that it is pure cold calling.

I am open to doing another house hack or purchasing a pure investment property and staying put in the first property. 

I think factory construction and the precise and pure angles etc is the way to go.So here is my issue.

That always makes it so much harder to rent, or secure a great tenant.Here's my standard go to list now for long term rental updates:-LVP that looks like wide plank white oak-Ultra Pure white on all the walls-Vertical/ modern stack backsplashes and showers-Pop of color on the cabinets (pop of color can include black too)-Butcher block countertops (lower end) or white quartz countertops (higher end)-Modern light fixtures in brushed brassKeep it neutral still but a couple of style choices with the light fixtures (which are cheap) and it will appeal to the masses.Good luck! 

It's a joy to be here.