Another reason for doing multiple smaller runs of the EMT is that there
is 2nd-story room where the cables run through tacked onto the wall,
it'd be nicer to have whatever conduit goes through that room to be as
unobtrusive as possible. I was thinking of multiple 1/2" EMT's up
against the wall, although maybe a single fat conduit might be less
obtrusive.

3" EMT seems to be about ten times more expensive than 1/2" EMT.
Although it undoubtedly could carry more than ten times as much RG-8
:-).

Instead of EMT you may want to use some of the plastic electrical conduit.
Not sure if it would meet the building codes for what you want by going from
one floor to the next. It is sort of a grey colored PVC type of pipe.
Probalby not really PVC but another plastic.
It will do nothing for the shielding, just may be neater to run than the
plain coax.