Will a newer Shimano series cassette be easily interchangeable?
The spacer is required for 11-34 cassette only, same story as CS-R8000 11-34. The 34t cog hangs over freehub body, which allows to use the good old 8-10 speed HG freehub body. For a note, Shimano MTB 11 speed cassettes do not require 11-speed specific freehub body either.




  My question is, will the spacer requirement on the new cassette with
  my road bike force me to mess with my rear derailleur indexing/etc.
  when I swap the wheelset/cassette?


To be honest, I did not run this cassette, but it would make no sense to change offset just because it uses a carrier spider instead of cogs riveted together. At worst, you'd have to put a thin spacer under the cassette when swapping it between 11-34 and smaller range options like 11-28.