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. 

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. 

I have a long-term rental in STN MTN (Stone Mountain) GA, and have a one year lease with a section 8 tenant and content with the cash flow, however, I can make more if the basement gets converted to a separate unit.

-we were quoted some private flood insurance around ~$1800 for what we thought was adequate coverage for all buildings, underwrote that number, and moved onA little further along we learn the flood zoning is zone AE, private flood insurance isn't much of a thing for an AE zone, and FEMA flood insurance is expensive. 

Aaron,No you will not find a Bank/Lender that will go into 3rd lien position if you if did I would not want to see that rate in terms of "High".