Without going into detail, my gut instinct is that while the Ahch-To site is an older mainline Jedi site, it was abandoned at some point well in the past, and reoccupied not less than a thousand years ago by a splinter sect -- probably in Yoda’s padawan days or early knighthood, actually -- and that’s when most of the construction actually occurred. The iconography of the Prime Jedi mosaic isn’t in line with any of the older Jedi iconography we’ve seen at other ancient temple sites: what it looks like to me, gut instinct, is something designed by someone who came out of the mainline (Coruscant) Jedi Order being very self-righteous and very literal about balance in the Force existing within an individual, not in the Force itself. Since we see the Jedi symbol being used in the Ahch-To sacred texts, but not in the older temple sites (Ilum and Lothal), this indicates that those texts date from the period in which the symbol was in use, suggesting a later date. It also does not boast the kind of colossal sculpture that we see on at a number of other Jedi or former Jedi sites, meaning that it either was not in occupation at the period those were constructed or modified, or it was occupied by individuals who didn’t go in for that. (Lothal also does not have colossal sculpture; I suspect the Lothal temple wasn’t in use then. I suspect the Lothal temple went through a series of abandonments and reoccupations over the course of several millennia before finally falling out of disuse several generations before the Clone Wars.)
