Grinders are fast but dusty as hell. You can hold a vac behind them, but its no easy job when you have that many m2 to take out. Too easy to damage tiles. Be sure not to go to deep, to avoid damaging any UFH or waterproofing membrane.
I have used a fein with the carbide tip blade. It was easy and safe to use, but not as quick as some of the youtube videos make it out to be. I guess it depends on the hardness of the grout. Also with the carbide blade it wore out after a few m2, (150mmx150mm
tile) so if you have 18m2 go for the diamond blades, apparently longer life. But its not a cheap option. And still abit dusty.
Grout colourant, looks like a good product. A lot less labour intensive, no chipped or scratched tiles and should be easier to get an accurate price on the job