We have a single-family long-term rental in Port Charlotte that we'd like to keep the current mortgage rate, but we do have some equity we'd like to have access to. 

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.

Own a few long term rentals. 

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...

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.

In this case, you might want to mix it up and be partially a mid-term and short-term rental.

House hacking strategies include living in one unit, renting out the others, renting out rooms, and mid-term rentals.

Have you made some progress in terms of getting some deals?

If it's less, buy short-term debt and some equities.