How plausible is a transoceanic voyage of a group of semitic people
from the Arabian Peninsula, beginning from say, Wadi Sayq, to the New
World, say Guatemala, around 600 BC?

The Mormons believe that ancient Israelites traveled from the Middle
East and settled in the New World. There are many online discussions
of this.

The ancient Phoenicians from the coast of present day Israel managed
to circumnavigate Africa. Modern vessels as small as 30 ft. have
circumnavigated the world, some even solo. To cross the distance from
the Med. to South america is at most a couple of months travel...It
all depends on the weather. If the general weather was mild in 600
b.c. there is no other reason why they could not have achieved this.
Also, there is the evidence of pyramid building. It cannot be ignored.

On the other side of the globe the Polynesians made long ocean going
journeys so technology of ships is not the biggest issue. The
difficulty is that in the Mediterranean it is not clear whether that
the navigation skills were up to the task. circumnavigating a large
land mass like Africa does not require any particular navigation
skills. "Keep the big land mass on your left".