Just out of curiousity why would someone choose this model over a short term rental that is isolated by itself (privacy) or a hotel? 

Term life insurance.

I never use the term 'rich' and honestly don't feel we are. 

Income is $600,000 per year and husband and wife both work full-time non real estate jobs.
Is it true that the tax benefits are greater, or realized more quickly, on purchases like furniture or televisions to itemize...

Own a few long term rentals. 

If you are just starting on your journey, it is best to start simple and then add complexity with experience. 

I'd prioritize finding long-term, reliable tenants who will treat your property with care over the quick turnover of short-term rentals.

-If you do a long term rental, how do you pay back the hard money lender/private lender (if you use one)?

Are they all long term?

I've included a preliminary floorplan below.My goal with this ADU is flexibility in terms of renting it out either short-, medium- or long-term rental in the future for income, as well as maximizing resale value (main property is 1300 sq ft 3/2) to someone probably with an investor mindset who would want to house hack.My main concern is whether it's a good idea to have 3 en-suites at the expense of having a small living room/kitchen area.