What is Reddit's opinion of BGTEC Dry Diamond Drill Bits 1pc 1/4" with Quick Change Hex Shank for Granite Marble Porcelain Tile Ceramic?

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I say go for it. Might be a good idea to get a diamond drill bit and drill the corners before you cut, it will make it a lot easier to manage the inside corners cleanly.

IME, the best way to keep dust down is to get a big utility sponge like the kind you'd use for tiling, get it moderately wet, then have a helper hold it against the blade as you cut. Plug the angle grinder into a GFCI outlet for safety. It will wick water into the cut as you go and turn much of the dust into a slurry that doesn't get up in the air nearly as much. The holder may have to adjust the pressure a bit, you don't want water pouring into the cut -- start with basically no pressure and increase until the dust mostly goes away.

EDIT: on a horizontal cut like you are talking about you might want to configure everything so that gravity is on your side -- sponge above.