That's some nice work on that piece of stone!

I have 8 lasers engravers; four of them C02, and four fiber lasers-
My BIGGEST C02 is a Chinese Triumph 1390, 80 watt RECI tube with a 51" x 35" working area- as big as it is, it obviously won't handle the size of black granite you're hoping to get done...

I COULD do it sideways as I can theoretically engrave 51" x any length.

--However--

I've been engraving for over 50 years, all of them using rotary tool machines- via pantograph machines until 1981 when computer controlled rotary machines became available, I've been using C02 lasers for 23 years, and fiber lasers for 7 years now... and I can say that in MY experience, trying to engrave anything similar to what I see in your sample pic, in anything BUT a one-and-done pass, it would be nearly impossible to align the sections together accurately enough to be 'seamless'. My big machine has a pass-thru slot, so I can 'theoretically' engrave 35" x 'any length', however-- 2x35" is only 70" of a 72" area, so if the entire plate is to be etched, it would have to be done in THREE sections with TWO seams to align....

Finding some one or some place with a large enough machine to accommodate that large of piece of stone, AND be capable of actually 'etching' the stone, is going to be your biggest obstacle, IMO... Most huge lasers are equipped with very high-powered lasers, designed for cutting, as opposed to engraving/etching...

Just found this online: https://www.lasermencnc.com/datail_14_61.html -- seems they sell 'low' powered LARGE lasers, 2 of the 3 shown on their webpage, a 1300x2500 and a 1500x3000 would be large enough- and they're available with tubes as low as 80 watts, which is definitely within engraving AND cutting power range... no clue as to price, but someone WITH one of these machines could likely help you, I would think!

Good luck-