I'd hardly call a huge order with dominion over a whole planet and two moons, deposited there by giant mysterious ships of unknown origin to be "a humble beginning". There was very little substantive difference between the Je'daii and the Jedi we're familiar with was largely cosmetic.
Anyway, according to the version of events presented in the TPM novelization (presumed to be straight from Lucas) the Sith breakaway was a relatively recent thing, compared to the founding of the Old Republic. More current material like 'Rebels' appears to have pushed it back a little, but no more than the 4 or 5 thousand years that it also was in the EU. either way, the Sith did not predate the Old Republic.
Also there's very little evidence to suggest the Order itself was directly involved in the Republic's founding. It's mostly an assumption based on them being there more or less from the beginning. For all we know they could have existed long before the Old Republic began to form around the core worlds, or they came into being centuries after it's founding and only later adopted as it's guardians.
Maybe in that ancient time there were many such orders or force worshipping knights in the galaxy, one or more of which already pledged to serve the fledgling Republic. The likes of the Guardians of the Whills could just be a fraction of what once was and then only a pale shadow of what they once were.
Again, I don't see why it's so difficult to grasp that these sites are places where the force is naturally strong. It would make sense that over time various religions and orders might occupy, abandon and rediscover such sites many times over the millennia. We know the Sith once ruled the galaxy and it stands to reason they did so from Coruscant until their infighting finally wiped them (almost) all of them out. We even know the temple was sacked when the Old Republic fell. It stands to reason than when the Jedi returned the sealed away the chambers and shrines the Sith had built, believing the darkness to have been neutralised. Then subsequent generations built over them in return, until they were eventually consigned to history and forgotten.
Anyway, according to the version of events presented in the TPM novelization (presumed to be straight from Lucas) the Sith breakaway was a relatively recent thing, compared to the founding of the Old Republic. More current material like 'Rebels' appears to have pushed it back a little, but no more than the 4 or 5 thousand years that it also was in the EU. either way, the Sith did not predate the Old Republic.
Also there's very little evidence to suggest the Order itself was directly involved in the Republic's founding. It's mostly an assumption based on them being there more or less from the beginning. For all we know they could have existed long before the Old Republic began to form around the core worlds, or they came into being centuries after it's founding and only later adopted as it's guardians.
Maybe in that ancient time there were many such orders or force worshipping knights in the galaxy, one or more of which already pledged to serve the fledgling Republic. The likes of the Guardians of the Whills could just be a fraction of what once was and then only a pale shadow of what they once were.
Again, I don't see why it's so difficult to grasp that these sites are places where the force is naturally strong. It would make sense that over time various religions and orders might occupy, abandon and rediscover such sites many times over the millennia. We know the Sith once ruled the galaxy and it stands to reason they did so from Coruscant until their infighting finally wiped them (almost) all of them out. We even know the temple was sacked when the Old Republic fell. It stands to reason than when the Jedi returned the sealed away the chambers and shrines the Sith had built, believing the darkness to have been neutralised. Then subsequent generations built over them in return, until they were eventually consigned to history and forgotten.