It could be, depending on where you land on the DIY vs building code spectrum.... Also what your local building code details in terms of low voltage cable.

In my experience unless you're intentionally going through air handling ductwork, CMG is fine, though I would go with CMR at least, just as a matter of safety. I have yet to see a residential home with any fire protection or fire breaks, at all, nevermind where you would think to put them, like between floors and whatnot. The homes in my area are mostly built from timber, so we're basically living inside a framework of dried out wood, so putting a fire break between wood framed floors where the floor/ceiling shares a set of wooden cross members for support, seems like it would be unnecessary, since the framing of the building is going to conduct the fire between levels.

Ethernet is low voltage, so it's largely unregulated. The only real regulation is regarding plenum or riser spaces, and the latter is mostly when floors are intentionally or naturally separated in terms of shared fate when it comes to fire. I only see fire breaks in concrete structures, usually apartments or commercial buildings.

YMMV, not every country has the same building codes, so every person reading this will need to do their own research or pay someone who knows.

My point remains, regardless of all this conjecture: even if it's $2000 or more, this is structure cabling that should service the premise for as long as it continues to stand, which is hopefully many decades. Over 20 years, at $2000 for installation, you're looking at a cost of around $8.34/mo for the cabling to exist.

Considering people will pay 10x that for Internet service, and the addition of Ethernet in the building will allow them to take full advantage of the internet they're paying for, I'd call that a bargain.