After further review, I may be mistaken about the ADS schedule. I would say there is nothing particularly special or odd about the rental scenario (e.g. an electing real property trade or business). My initial read on IRS p946 p.27 was that listed property utilized less than 50% for business use must use the ADS schedule. However, I now see that "listed property" has a specific meaning that doesn't look to include this kind of rental property. Further, if I utilize 20% of the cost at 100% business use, it would no longer fall under that category anyway. So I believe GDS is the correct path here instead.
https://www.irs.gov/pub/irs-pdf/p946.pdf
