you can only understand Plato within the greater context of Greek culture, and particularly if you're trying to make a case for astrological interpretation then one has to study the greater corpus of Greek mythology and cosmology...ostensibly Plato is not concerned with cosmology at all.
The original premise for Atlas, and the associated pillars of Hercules is sourced from other than Plato, dismiss such and any theory is baseless.
In any case much of Homer's work which you appear to have confused with that of Plato can be interpreted as being astronomical in origin. For an explanation of that ... which I admit is not generally accepted but is perfectly logical, and which I believe is what Homer was presenting in allegorical form, see Homer's Secret Illiad by Florence and Kenneth Wood. This was published in 1999 by John Murray.
Homer is only one of any number of Greek sources for cosmology/mythology, which would provide source background for Plato's world view, i see no justification for attempting to extricate Plato from greater Greek culture.
All theories that disregard the Greek conception of Atlas and the Pillars of Hercules sink...