this came up some years ago at a local public school, about their
kitchen. they had fire marshalls, firemen, architects, insurance reps,
it became a circus.

final conclusion, elbows are fine. they had 8, kitchen was a interior
one.

leave some access areas, and blank covers so you can occasionally
inspect the inside of the line and clean if necessary.

I like the multi speed fan, low for steam or minor odors, high for
frying.

have you considered down and out thru the basement?