as you say there are only a few Robt Garretts born in that period - you said that his MI gave age 61 which gives birth c. 1730 - the nearest is Robt born to Wm + Mary caley but he is not mentioned in her will and there is a smallpox death of an infant Robert garrett which is most likely he (needs checking against burial reg).
There is also robt c/o wm + margt goldsmith bapt 1721.
Maybe a tedious transcription search of all Wm garrett wills - margt goldsmith may have remarried and her will filed under a different name.

There is also the possibility that he was illegitimate but I can't see any obvious contender